What 19 of the most successful people in the world were doing in their teens and early 20s

Before Mark Cuban was on "Shark Tank", he was just starting out in the business world. A recent graduate of Indiana University, Cuban began working at Mellon Bank. However, even in his entry-level position, Cuban wanted to prove his worth and go above and beyond.

"I wanted to be more entrepreneurial. I took the initiative," he wrote in an essay for Forbes. "I used to send notes to the CEO of the bank. I once cut out a magazine story about how corporations could save money by withholding Social Security and sent it to him. He sent me a thank-you letter back."

However, Cuban's initiative began to be seen as a threat to his superiors. After his boss at Mellon Bank told him "never to go over him or around him, or he'd crush" Cuban, the future billionaire decided to leave. He later moved to Texas, where he was fired from a computer software company for closing a deal without the CEO's approval. He went on to start his own company, MicroSolutions, which was eventually sold to CompuServe for $6 million.

At the young age of 26, Kim Kardashian's star was on the rise. As a close friend of Paris Hilton, Kardashian appeared on the hotel heiress's hit TV show, "The Simple Life", in 2006. However, her role on the show revealed more about the now-media mogul than just her friendship with Hilton. In her brief appearance on the show, Kardashian is seen helping Hilton organize and clean out her closet, which she made money doing at the time, along with reselling designer goods on eBay.

"My closet business came about when I was at my godparents' house, Sugar Ray Leonard and his wife Bernadette," Kardashian explained in an interview with now-defunct Player Magazine in 2006. "Bernadette's closet was massive and had so much stuff in it. I said to her, 'You really need to clean out your closet.' Well, we spent the whole night doing that."

Kim Kardashian West is said to have organized the closets of high-profile celebrities such as Kenny G, Rob Lowe and his wife, Cindy Crawford, Serena Williams, as well as close friends like Nicky Hilton and Nicole Richie.

Soon, Kardashian became known as the "Queen of the Closet Scene," and when asked about her future goals, Kardashian quipped that a reality tv show about her closet business would be exciting. Little did she know she would become one of the most recognizable people in the world with a net worth of $72 million.

Even at the young age of 17, Serena Williams was a force to be reckoned with. Though Williams was not yet a legal adult, she was nevertheless ranked as the No. 6 tennis player in the world and was about to secure her very first Grand Slam title.

On September 11, 1999, Williams was slated to go up against four Grand Slam champions. In a huge upset, Williams was able to defeat all of them, winning the title of Grand Slam champion in the US Open. She was the first African-American woman to win a major singles championship since Althea Gibson in 1958.

When Ellen DeGeneres was in her early 20s, she suffered a heartbreaking loss that would inevitably lead to her stand-up career and future success. When DeGeneres was 20 years old, her then-girlfriend passed away in a tragic car accident after the recently broken-up couple went to see Ellen's brother's band play at a local gig. Speaking about the accident now, DeGeneres reveals she felt a tremendous amount of guilt over what happened.

"She was trying to get me to come back home. I acted like I couldn't hear her because the music was too loud," Ellen said in an interview featured on Oprah's Master Class. "I was being really...dismissive of her."

Later that night, DeGeneres' girlfriend would be involved in a fatal car accident. Ellen returned home to her apartment and began writing, trying to channel her grief.

"The basement was infested with fleas. There were just fleas everywhere, and I remember thinking, 'It just seems so ridiculous that this young, 23-year-old girl who I was just living with [is] gone, and fleas are here,'" she said. "I thought, 'It would be great to just pick up the phone and call up God and talk about this.'"

Ellen DeGeneres' stand-up bit titled "Phone Call to God" would become her gateway to fame. Several years after the accident, she performed it during her first network television appearance on Johnny Carson's "The Tonight Show." Carson was so impressed by her performance that he invited her to sit on his couch; she became the first female comic to be invited to do so after performing a stand-up routine on the show.

JK Rowling is best known as the genius behind the "Harry Potter" series, but she didn't come up with the idea for Harry, Ron, and Hermione until she was 25. She struggled to get the book published at first, and couldn't focus at work, which led to her being fired from Amnesty International.

According to The Daily Mail, Rowling spent her teenage years going to festivals and hitch-hiking around the UK.

Microsoft co-founder Bill Gates discovered his love of computers at age 13 while attending prep school in Seattle. There, he wrote computer code for a version of tic-tac-toe. He then met and went into business with Paul Allen, his Microsoft cofounder, according to Biography.com.

Gates attended Harvard University but dropped out at age 20 in 1975 to focus on Microsoft, which then made him the world's richest self-made billionaire.

Jeff Bezos, the founder and CEO of Amazon, started his professional career in McDonald's when he was a teenager. According to Cody Teets, author of " Golden Opportunity: Remarkable Careers That Began at McDonald's," he wasn't very good at it.

Bezos told Teets: "My first week on the job, a five-gallon, wall-mounted ketchup dispenser got stuck open in the kitchen and dumped a prodigious quantity of ketchup into every hard-to-reach kitchen crevice."

"Since I was the new guy, they handed me the cleaning solution and said, 'Get going!' I was a grill man and never worked the cash registers. The most challenging thing was keeping everything going at the right pace during a rush."

Tina Fey, the brains behind "Mean Girls" and "30 Rock," was really into theatre at college, but she didn't get into comedy writing until years after she graduated.

First, Fey moved to Chicago so she could hang around acting workshops, and in her early 20s, she worked as the childcare registrar at a YMCA.

She then joined the improv troupe Second City because she "knew it was where a lot of SNL people started," according to The New Yorker. She was hired by "Saturday Night Live" producer Lorne Michaels as a writer in 1997.

By the time the world's most famous investor was 16, he had earned today's equivalent of $53,000 (£41,000) according to the biography "The Snowball: Warren Buffett and the Business of Life" by Alice Schroeder. One of Buffett's first jobs was delivering The Washington Post as a paperboy. He also sold golf balls and stamps, buffed cars, and set up pinball machines in barbershops.

He was rejected from Harvard Business School, but then attended Columbia Business School and worked as an investment salesman, securities analyst, and stockbroker.

Oprah Winfrey is now one of the most famous talk show hosts, actresses, and producers in the world, but she realized she loved media at age 14 when she moved to Nashville. She found her first job at 16 as a broadcaster for WVOL, a Nashville radio station.

At age 19, as a sophomore at Tennessee State University, Winfrey left school to start her media career. However, she had a bumpy ride into fame after being fired from hosting the 6 p.m. news on Baltimore's WJZ-TV in 1977.

"I had no idea what I was in for or that this was going to be the greatest growing period of my adult life," Winfrey told the Baltimore Sun. "It shook me to my very core, and I didn't even know at the time that I was being shaken."

Jay-Z adopted his rapper name at age 20, but was born as Shawn Corey Carter. He chose the name partly because it was similar to his nickname "Jazzy," partly as a tribute to his mentor, rapper Jaz-O, and also as a reference to the J/Z subway station near his home in Brooklyn.

For a few years, Jay-Z was performing alongside other rappers, but he remained fairly anonymous according to Biography.com. Afrizap claims he even sold CDs out of his car. He and two friends, Damon Dash and Kareem Burke, founded Roc-a-Fella Records in 1996, which is when Jay-Z started to be recognized as an emerging rap star.

The former president lived in Honolulu for most of his childhood. At school, he was skilled at basketball and graduated in 1979 at age 18 with academic honors. He was one of only three black students at the school, Punahou Academy, which is where he became particularly aware of racism and what it meant to be African-American, according to Biography.com.

He then went to Harvard Law School, where he became the first black president of the Harvard Law Review. He later became the 44th President of the United States.

In her 20s, co-founder of The Huffington Post and businesswoman Arianna Huffington was studying economics at the University of Cambridge. There, she became the first foreign, and third female, President of the Cambridge Union.

At age 21 she met British journalist Henry Bernard Levin while on a panel for a quiz show. He became her mentor while she wrote " The Female Woman," which was published when she was 23. She and Levin then traveled the world for a few years together, attending music festivals.

Morgan Freeman is now one of the most famous and sought after actors in Hollywood. However, this wasn't always the case. Freeman worked very hard to get to where he is now.

According to Biography.com, he joined the Air Force after high school to become a fighter pilot. Though he loved acting, it wasn't easy breaking into the industry, and Freeman spent much of his 20s struggling to find anything more than limited success.

Arguably, his career-changing role was in "Driving Miss Daisy," which he was cast in at 52 years old.

Facebook CEO and co-founder Mark Zuckerberg got into building computer programs at a young age. At age 12 he used Atari BASIC — a programming language — to create a messaging program that he called "Zucknet." In his early teens at high school, Zuckerberg built another program called Synapse which learned your music taste, according to Funders & Founders. Microsoft offered to buy it for $1 million (£772,857), but he declined the offer.

In high school, he also learned to read Hebrew, Latin, and Greek, before he was accepted into Harvard University. Facebook was born after he was approached by the Winklevoss brothers. Zuckerberg was able to build the program in a week. He dropped out during his sophomore year to commit his time to Facebook.

Richard Branson is the founder of the Virgin Group, which owns over 200 companies in more than 30 countries worldwide. He started his first company when he was just 17 years old after dropping out of school at age 16, according to Biography.com.

He struggled with academia, but not with business, and founded youth culture magazine "Student" which sold $8,000 (£6,183) worth of advertising in its first edition.

Two years later, Branson started selling records via mail, which turned into a record store, which then turned into a recording studio called Virgin Records.

Presidential candidate Hillary Clinton was destined for big things from an early age. She gave the commencement address at her graduation from Wellesley College in 1969 at the age of 22 and later attended Yale Law School.

At University, Clinton worked at the Yale Child Study Center where she took on cases of child abuse at Yale-New Haven Hospital. She also volunteered at New Haven Legal Services which provided legal advice to people who couldn't afford it.

At 23, she started dating Bill Clinton. She went on to become First Lady of the United States of America, Secretary of State, and the 2016 Democratic presidential candidate.

Murray started playing tennis at just three years old. By age eight, he was competing against adult players. At age 18, tennis champion Andy Murray rose 287 places in the world rankings. He was also starting to grow his hair very long.

"At the start of the year, I said I would be in the top 100 and a lot of people didn't think I could do it, " he said at the time, according to The Herald. "But I always thought I would. So I've proved a lot of people wrong. Now I've been home for a while, I've had the chance to look around and see what's been happening these past few months. I've been pretty impressed, and I just hope I can continue that next year."

In January 2019, Murray announced he would be retiring. He has won two Olympic tennis singles titles, three Grand Slams, and was the winner of the 2016 ATP World Tour Finals – making him the No. 1 tennis player in the world at the time.

At age 12, the PayPal, Tesla Motors, and SpaceX founder Elon Musk had written code for a space-based video game called Blastar. In 2015, a software engineer at Google turned it into a working game, according to The Verge.

At age 17, Musk moved to Canada to attend Queen's University. Shortly after, he moved to the University of Pennsylvania to study business and physics. He then pursued a Ph.D. at Stanford University in energy physics, but he dropped out after just two days to become part of the "internet boom" in the 90s.

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