Watts and her ex, actor Liev Schreiber, share two children together.
In the sweet post, the Mulholland Drive star posted a carousel of photos showing Alexander "Sasha" Schreiber looking like quite the young man while dressed in a beige suit and white dress shirt. The photos also included a seaside outing and of course, two adorable photos of Sasha holding a baby bottle as well as being held as a baby by his doting mother.
In her caption, Watts expressed how fast time flies.
"Happy 16th to my darling boy. I cannot believe how fast this happened?!" she wrote in her caption. "Beyond proud of the young man you've become with the most wonderful spirit, fantastic sense of humor and kind, compassionate soul. So lucky i get to be your mum. Thank you for still letting me hold your hand sometimes. Even though i know you probably just forget to pull it away. You blow me away!! 🤦♀️Happy birthday to ❤️🎂🎉 @sashapeteschreiber."
She also took to her Instagram Story and posted a candid photo. She overlayed the text with, "Happy birthday to my darling boy @sashapeteschreiber."
Not to be outdone, the Ray Donovan star, who also shares 14-year-old daughter, Kai, with Watts, took to Instagram and posted a side by side photo that he captioned, "Happy Birthday Handsome! @sashapeteschreiber."
In Watts' comments section, their celebrity pals also expressed similar sentiments. Gwyneth Paltrow commented, "WHAT. This is nuts." Julianne Moore added, "Happy birthday Sasha!!! ❤️" and Paulina Porizkova wrote, "What a lovely photo of you two."
Molly Ringwald added, "Happy birthday! What a beautiful young man. 👏"
The birthday celebration comes just over a month after Watts and Billy Crudup's surprise wedding in New York City.