Some stars seem older just because they've been around forever, while others are aging because they've been living the glamorous life a little too hard.
Still, a lot of celebrities have the cutest little baby faces or they take really good care of themselves.
Selena Gomez comes across as fun and approachable. Meanwhile, Demi Lovato always seems so pulled together and wise beyond her years on "X Factor." Who do you think is older?
Selena's older! Both 20-year-old stars were born in 1992, but Selena's birthday is July 22 while Demi celebrates hers on August 20.
Angelina Jolie and Alyson Hannigan were born in the 1970s. Do you know which actress came along first?
Alyson did! The actress is doing something right because she just had a birthday on March 24 and looks nowhere near 39! Angelina will celebrate her 38th birthday on June 4.
Kristen Stewart favors a mature, serious look on the red carpet while Snooki likes to have fun. But which one is older?
Snooki is already 25. Kristen? She turns 23 on April 9. Waaaah!
Both Ethan Hawke and Jason Bateman have been in the business for a very long time. But who's been on earth longer?
Reality bites! Jason turned 44 in January, making him almost two years older than Ethan.
Mariah Carey signed with Columbia Records when she was 18 while Hilary Swank had her breakout role in 1994's "The Next Karate Kid." Who got started first?
Mariah takes the birthday cake! The singer just turned 43 on March 27. Hilary Swank will be 39 this summer.
Harry Styles and Justin Bieber are still teens. It's debatable who has won the most teenagers' hearts, but there's a clear winner on who's older.
Harry turned 19 on February 1—just one month before Justin!
Michael Buble and Justin Timberlake are both admired for their smooth moves and impressive vocals. Which do you think rocks the title of older singer?
Michael, 37, is older than Justin—to the tune of five years!
One is almost 25 while the other is 26. Can you figure out if Adele is older or younger than Kesha?
Adele will turn 25 in May, though her sophisticated and sometimes matronly looks would indicate she's much older—and perhaps wiser—than 26-year-old Kesha.