Bill Belichick became the head coach of the Cleveland Browns in 1991, at age 39.
Belichick, 66, is going on his 20th season with the New England Patriots.
Gregg Popovich was the San Antonio Spurs GM before becoming the head coach in 1996, at age 48.
Now 70, he's been with the Spurs ever since.
Mike Krzyzewski was 33 years old when he became the head coach of Duke men's basketball in 1980.
Now 72, "Coach K" is perhaps the most well-known coach in college sports.
Joe Maddon was 52 years old when he took over the Tampa Bay Rays in 2006.
Now 65, he's on his fifth season with the Chicago Cubs.
Nick Saban was hired by Michigan State in 1995 at 44 years old, re-joining the college ranks after four years with the Cleveland Browns.
Now 67, Saban has become a legendary coach at Alabama over the last 12 years.
Mike Tomlin got the Pittsburgh Steelers job in 2007 when he was 35 years old.
He's been with the Steelers since and is now 47.
Mike D'Antoni became head coach of the Phoenix Suns in 2003 when he was 52.
Now 67, D'Antoni is on his third season coaching the Houston Rockets.
John Tortella took over the Tampa Bay Lightning in 2000, at 42 years old.
Now 60, Tortorella is with the Columbus Blue Jackets and one of hockey's most well-known coaches.
Terry Francona became the Philadelphia Phillies manager in 1997, when he was 38 years old.
Now 60, Francona has been with the Cleveland Indians for seven years.
Pep Guardiola was named FC Barcelona's manager in 2008, when he was 37.
Guardiola is 48 and on his third year with Manchester City.
Pete Carroll returned to the NFL in 2010 and became the head coach of the Seattle Seahawks at 59 years old.
Carroll is 67 and still with the Seahawks.
Doc Rivers was 38 when he got his first job as a head coach, with the Orlando Magic in 1999.
Rivers has coached every year since then, now with the Los Angeles Clippers at 57 years old.
Barry Trotz was 36 when the Nashville Predators hired him as their head coach in 1998.
Trotz is now 56, in his first year with the New York Islanders.
Tom Izzo has been Michigan State basketball's head coach since 1995, when he was 40 years old.
25 years later, Izzo is 65 and still with the team.
Jim Harbaugh's first head coaching job was with Stanford in 2007, when he was 43.
Now 55, Harbaugh is the head coach of Michigan.
Jurgen Klopp was 34 when he became the manager of Mainz 05 in 2001.
Klopp, now 51, has been at Liverpool since 2015.
Ned Yost was 48 when he was hired as the manager of the Milwaukee Brewers in 2003.
Yost is now 64 and with the Kansas City Royals.
John Calipari's first coaching job was at Massachussetts in 1988, when he was just 29.
Calipari, 60, is in his 11th season with Kentucky and has a lifetime deal.
Andy Reid was 41 when he became head coach of the Philadelphia Eagles in 1999.
Now 61, Reid is the head coach of the Kansas City Chiefs.
Rick Carlisle began his head coaching career in 2001 with the Detroit Pistons when he was 42.
Carlisle is now the second-longest tenured coach in the NBA next to Gregg Popovich, now 59 and in his 11th season with the Dallas Mavericks.
Mike Babcock was 39 when he became head coach of the Mighty Ducks of Anaheim in 2002.
Now 55, Babcock has been the head coach of the Toronto Maple Leafs since 2015.
Bruce Bochy was hired as the San Diego Padres manager in 1995, when he was 40.
Bochy, 64, has been with the San Francisco Giants since 2007.
Roy Williams' first head coaching job was with Kansas in 1988, when he was 38.
Williams is now 70 and has been with UNC for 16 years.
Now, check out how some of the NBA's biggest stars have changed...
