10 things you'll regret doing in your 20s

How do you know if you're taking full advantage of your 20s, making all the right decisions in your personal and professional lives?

Well, you don't. Life is about taking chances and doing your best.

But those who've already been through this critical decade can certainly help point out the landmines. That's why we turned to threads on Quora and reddit, where users weighed in with their biggest regrets from their 20s.

Here are a few things you might want to avoid:

1. Not exercising

Quora user Carl Logan regrets never working out in his 20s.

"If I [had] hit the gym I probably would've been a lot happier and would've had more success with the opposite sex," he writes.

Even beyond happiness and the ability to attract mates with your six-pack abs, regular exercise in your 20s can help prevent health issues down the line. One recent study found that your fitness level in your 20s may have a major impact on your risk of heart disease and death as you progress toward middle age.

2. Worrying about what other people think

"I wasted a lot of time worrying about what others think - I've learned it rarely matters," Logan says.

In fact, research suggests that people generally overestimate the amount of attention others pay them. It's called the spotlight effect, because people mistakenly believe they are the center of attention in a room.

If you accept this idea in your 20s instead of later on, you'll have more time to act freely, without fearing that you look like an idiot.

3. Letting your parents' opinions determine your life choices

Riina Rinkineva says she regrets "not standing my ground against my parents for what I wanted for myself in my life and what I didn't want."

It's incredibly important to set some boundaries between you and your folks, so that you have space to figure out what you want personally and professionally.

At the same time, you shouldn't cut yourself off completely from parental support. As psychologist Jeffrey Arnett told Business Insider, parents "often have life experience and wisdom that you haven't acquired yet."

4. Racking up credit-card debt

Yash Mishra says he regrets getting a credit card and "charging like crazy" in his 20s.

If you find yourself in a similar situation, consider taking a tip from one Business Insider reporter and going on a cash-only diet, during which you stop using credit and debit cards completely.

5. Getting married too quickly

Before getting married, Diane O'Neil says, "I should have first found out who I was and what I was capable of achieving as an individual; I became someone's wife long before I found out what I wanted to do personally."

Obviously, everyone is different and for some people, getting hitched before 30 is the perfect choice. In fact, some research suggests that people who marry in their mid to late 20s have happier marriages than those who marry later.

But if you feel like you need more time to explore - by traveling, trying out different careers, and learning what you want in a romantic relationship - you may not want to rush to the altar.

6. Not investing

One of Ramya Sridharan's biggest regrets from her 20s is that she didn't invest in the stock market. As she notes, the earlier you start investing, the higher the returns.

Sridharan is right - the earlier you invest, the more time your money has to accrue interest (it's a principle called "compound interest"). That's why you should open a retirement account, such as a company-sponsored 401(k) or an individual retirement account (IRA), as soon as possible.

7. Not traveling the world

"The biggest regret I have about one decision I made in my 20s is not traveling enough when there were a lot of opportunities," writes Vishnu Prabhu.

Inspired to globe-trot but overwhelmed with all the potential places to visit? We've narrowed it down to 40 trips you should take before you turn 30 - from swimming with jellyfish in Palau to camping out in Nevada for Burning Man.

8. Not mustering the courage to ask someone out

Over on Reddit, stardust7 says, "I regret not being more direct when I liked someone. I had no confidence back then."

If you're not quite bold enough to approach the object of your affection in person, be slightly less bold and take these tips from comedian Aziz Ansari on how to text them. (Hint: "heyyy" probably won't work.)

9. Forgetting to floss

"I have cavities now because I didn't floss daily," says Eurycerus. "Kills me that I could've prevented it. Now I floss daily."

But it's not just cavities that result from letting plaque build up between your teeth. Medical experts say that not flossing can cause periodontal disease - when the gums recede and create a gap between the gum and the tooth that can become infected.

Periodontal disease is linked to other issues as well, including heart disease, HPV infection, mouth cancers, diabetes, and kidney failure.

10. Never living alone

"I always had a roommate or lived with my fiancé/wife," writes an anonymous redditor. "I strongly believe I missed out on discovering some level of self sufficiency. … It's not the [worst] thing in the world, but I feel like it could have contributed to my growth."

Living alone is getting more common - as of 2013, as many as 23% of Americans were doing it, partly thanks to the decline in marriage rates. But there's still a relative paucity of advice for singletons.

Kate Bolick, author of "Spinster," tells Business Insider that, if you're living alone, it's important to accept that you'll be lonely at times and there's nothing shameful about it. You'll also want to place special importance on your friendships, which can have a big impact on your health and happiness.