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See Reese Witherspoon's sentimental tribute to daughter's 21st birthday

The actress sent her mini-me a sweet message on Instagram.
/ Source: TODAY Contributor

Reese Witherspoon celebrated her daughter’s milestone birthday by sharing a snap of the lookalike duo on Instagram alongside a sentimental message.

“Wow! How is it possible that this little girl is now 21 years old?” Witherspoon shared in the post. “Happy Birthday to my sweet girl who has become the most incredible young woman. 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...”

It’s not the first time Witherspoon and Ava have shared “aww”-inducing social sentiments. Last year, Witherspoon called watching her daughter grow ‘the greatest gift’ and when Ava turned 18, she shared a photo of her famous mom’s thoughtful gift — 18 letters Witherspoon had written to her every year on her birthday.

The 21 year-old is in her third year at UC Berkeley, a transition that initially made her mom a little emotional.

“It’s weird when your children go away to college. It’s hard,” Witherspoon told Ellen DeGeneres in a spring 2019 interview. “... I might have gone into her empty room, laid down on her bed and cried when she went to college.”

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The Oscar-winner shares Ava and son, Deacon, 16, with ex-husband Ryan Phillippe. The pair met at the actress’ 21st birthday celebration in 1997 and married in 1999, the same year she gave birth to Ava.

“I got pregnant [with Ava] when I was 22. I delivered when I was 23. 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. Your entire life will change," Witherspoon said in a 2019 YouTube video posted to the Reese Witherspoon x Hello Sunshine YouTube channel that detailed why the mom of three is glad she had children when she was young.

Witherspoon is no stranger to sharing relatable mom moments on Instagram. Most recently, the 44 year-old joked that homeschooling was “going great” alongside a photo of son Tennessee, 7, who she shares with husband Jim Toth, hanging from the kitchen counter.

But today, Ava isn’t the only one celebrating a milestone.

“Congratulations on 21 mom years,” pal Jennifer Garner commented alongside 5 shooting star emojis.