Reese Witherspoon and Ryan Phillippe reunited to celebrate their son Deacon's 18th birthday.

The pair — who were married between 1999 and 2007 — both shared touching Instagram tributes to their son on his special day on Saturday (Oct 23) as they posed together with him to mark the occasion.

Ryan wrote on the image-sharing platform: "Happy 18th birthday to our gorgeous, smart, talented, and caring son.

"You are a true light in this world and are so loved by all who know you. We are lucky to be your mom and dad. Love you, pup..(I'd say we did pretty good @reesewitherspoon) (sic)"

The Legally Blonde actress shared a series of snapshots of her son over the years, including one of him getting ready to blow out the candles on his birthday cake.

Reese — who also shares daughter Ava, 22, with Ryan and son Tennessee, nine, with husband Jim Toth — captioned the pictures: "How did this happen?!! @deaconphillippe is 18?!!

"One day he was trading Pokémon cards, singing Bruno Mars songs, and playing American Ninja Warrior in the backyard."

"The next day, he is taller than me, cooking the family steaks on the grill and making his own music with his best friends. My heart is bursting with pride about the young man he is becoming. Happy 18th Birthday Deacon! I love you to the moon around the sun and all the stars (sic)"

Witherspoon revealed that the prospect of her children getting older made her emotional.

The Morning Show actress told People in June: "That makes me want to cry, the idea of having a long relationship with adult children.

"I never expected the kind of relationship that I have with them, but it's so rewarding to be able to have kids that you can process life with a little bit, and they help me understand the complexities of what it means to be a human now.

"So I'm really, really grateful that these little humans are in my life." — BANG SHOWBIZ

Photo: Ryan Philippe/Instagram