If you're looking at Reese Witherspoon wondering how she can possibly have a grown-up daughter, well, join the club. The actor's first child Ava Phillippe (who you might mistake for her twin) celebrated her 21st birthday last week - so yes, she's officially an adult and we feel old.

Appearing onThe Drew Barrymore Show, Reese recently opened up about becoming a mother at 22 - all while navigating life in the spotlight.

Watch: Some of our favourite celebrities on parenting. Post continues below.

"To be completely candid, I was terrified," she shared.

"I got pregnant when I was 22 years old, and I didn’t know how to balance work and motherhood, you just do it."

Reese is one of many A-listers who started a family in their early twenties. Put it down to growing up fast, but there's always been a trend in Hollywood for taking on milestones like marriage and kids (and let's be honest, divorce) relatively young.

From Hilary Duff to Kylie Jenner, here are 12 celebrities who had their first child before 25.

Reese Witherspoon

Reese gave birth to Ava Elizabeth Phillippe on September 9, 1999. At 22, her career was only just beginning.

"I've been a mum since 1999," Witherspoon said on Hello Sunshine. "To be totally honest, it was scary. I was scared. I didn't know what it was going to do to my job or my career."

Sofia Vergara

At 18 years old, Sofia Vergara married first love Jose Luis Gonzalez. One year later, she gave birth to her son Manolo Gonzalez Vergara.

"[He was] my high school sweetheart, the only one. We married and had a son, Manolo," she told Parade.

Manolo is now 28.