Neil Patrick Harris celebrates his twins' 12th birthday: 'Greatest two things that have ever happened to me'

Neil Patrick Harris shared an adoring message for his twins' 12th birthday on Wednesday.

Alongside a split photo of him posing with his daughter Harper and his son Gideon, both whom he shares with his husband David Burtka, the "Uncoupled" actor wrote about how much they changed his life.

"Twelve years old today. Harper and Gideon are, without question, the greatest two things that have ever happened to me," he wrote in the caption.

"They inspire me, make me chortle, allow me to be an imperfect father, and fill me with a type of love I never knew could exist," he continued.

Harris also wrote about how he and Burtka have embraced parenthood and their family.

"Being a parent is all-consuming, the most constantly challenging thing David and I have ever done. But so, so worth it," he wrote. "The[y] are remarkable. They are my children. They are my world."

Of his children, Harris added that he is "supremely proud of who they have become, of who they are becoming, and am just so grateful to have them in my life."

He ended his post with a message directly addressed to the twins.

"Happy birthday, Gid and Harps," he said. "I love you so much, and can’t believe you’re TWELVE!! How in the whenwhat?!? Xoxo balloon emojis #grateful #goldenbirthday"

In an interview with "Good Morning America" in 2019, the actor said he'd "always suspected" he would be a "family man."

"We did the surrogacy thing, which was awesome, and we tried for twins and we got twins and they're the most complicated things I've ever had to deal with," he told "GMA."

He also expanded on how he and Burtka aim to communicate with their children.

"I really appreciate that our parents talked to us like regular people and didn't talk down to us as little kids," he shared. "I was able to interact with adults when I was younger in a way that was a level playing field, and we try to talk to our kids the same way."