The 30-year-old actor said on Monday's episode of Watch What Happens Live that he texted Grint, 31, to congratulate him after Grint welcomed a baby girl with his longtime girlfriend, Georgia Groome.
"I texted him the other day, actually," Radcliffe said. "I'm so happy for him. It's very, very cool. It is also like super weird for me to think that we are all of the age where we're having children, but we definitely are."
"We're all just a terrible yardstick for how old you are," he jokingly added. "I remember when I turned 30 a lot of people in my life were really depressed to find out that that had happened."
Grint and Groome confirmed their daughter's birth May 7.
"Rupert Grint and Georgia Groome are delighted to confirm the birth of their baby girl," the couple's rep said. "We would please ask that you respect their privacy at this very special time."
Grint had expressed his desire to have kids in an interview with The Guardian in 2018.
"Turning 30 felt strange. It just doesn't feel like I'm there yet and I don't know what the future holds. I'm just going to go with the flow, keep playing interesting characters and see what happens. I'd like to settle down and have kids soon," he said.
Radcliffe and Grint played Harry Potter and Ron Weasley, respectively, in the Harry Potter films, based on the J.K. Rowling book series. Radcliffe and other stars, including David Beckham and Dakota Fanning, are reading the first Harry Potter book, Harry Potter and the Sorcerer's Stone, aloud chapter-by-chapter in a new online series.