Reese Witherspoon shared adorable photos of look-alike daughter Ava Phillippe to celebrate her 21st birthday

• Reese Witherspoon shared a sentimental tribute to celebrate her daughter Ava Phillippe's 21st birthday on Wednesday.
• The 44-year-old actress and producer posted a current photo of herself and Phillippe along with an old black-and-white photo of her daughter as a baby.
• "Wow! How is it possible that this little girl is now 21 years old?" Witherspoon wrote. "Happy Birthday to my sweet girl who has become the most incredible young woman."

Reese Witherspoon wrote a heartfelt tribute to celebrate her daughter Ava Phillippe's 21st birthday on Wednesday.

The 44-year-old actress and producer posted a recent photo of herself and Ava, whom she shares with her ex-husband and "Cruel Intentions" costar Ryan Phillippe, along with a black-and-white throwback picture of her daughter as a baby.

"Wow! How is it possible that this little girl is now 21 years old? Happy Birthday to my sweet girl who has become the most incredible young woman," Witherspoon wrote.

She continued, "Her kindness, her compassion, and her huge heart never cease to amaze me. Ava, there aren't enough words to describe how proud I am of everything you've already accomplished. I cannot wait to see ALL the Good you will put into this world. I love you so much."

Ava wrote back, "love you the most, mama!"

Viola Davis, Zoe Kravitz, Kerry Washington, and more stars also left comments wishing her a happy birthday.

Witherspoon had Ava, her eldest child, six months before she began filming "Legally Blonde" at 23 years old.

"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," she said during a Reese Witherspoon x Hello Sunshine YouTube video.

Becoming a mother at a young age caused her "entire life" to change.

"Everything you believe, every piece of food you eat, every piece of independence you have," she said, adding, "You can't go out without thinking of another person. You can't look at your groceries without thinking of another person. You don't think about whether you're cold or hot, you think about your kid and whether they're cold or hot. You don't sleep anymore."

In retrospect, however, Witherspoon said she's grateful to have had two of her kids, Ava and Deacon Phillippe, in her 20s.

"I find having little kids is more physical and it's just really hard on your body. So I am glad that I had kids kind of young," she said, admitting that having her third child, Tennessee Toth, with now-husband Jim Toth at 37 years old was "so much harder."

"This is just me being real. Everybody's life experience, you have to do when it's right for you."