EXCLUSIVE: George Clooney Dreads Turning 60, Celebrated 55 by 'Hiding in My Room and Crying'

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He's only 55, but George Clooney is already apprehensive about his 60th birthday.

ET caught up with the actor and his pal, Julia Roberts, at the Cannes Film Festival -- where they're promoting their film Money Monster, out May 13 -- and the Hail, Caesar! star candidly admitted that his next milestone birthday is the first one to really worry him.

Asked how he celebrated his big day on May 6, Clooney joked, "By mostly hiding in my room and crying because the next one is 60."

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"The thing is, age has never been an issue -- turning 50 didn't bother me, turning 55 didn't bother me," he explained. "None of it bothered. I think 60 is going to kind of.... *cough*."

"You have got time, though. It doesn't actually go 55-60," Roberts, 48, replied, giving him a hard time. "There are other years in between."

"Oh, I have been counting wrong," George joked. "Alright, I'll be fine."

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Leave it to a friend like Roberts to help you realize that age is nothing but a number, especially when you're George Clooney.

The five-time co-stars also talked about their recent epic session on "Carpool Karaoke," explaining that their decision to appear in the segment with a big-time singer like Gwen Stefani stemmed mostly from their lack of confidence in their own vocal cords.

"They asked us to do one, just to sing, and I said, 'Why don't we sing with somebody else because we shouldn't be singing,'" Clooney revealed of how the Late Late Show With James Corden appearance came about. "We have heard each other sing and we should not be singing."

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"Especially together," Roberts added.

"James came back with Gwen," Clooney continued. "And we thought that would be perfect."

"We were just like, 'Uh, does she want to do it with us?'" Roberts joked, before Clooney added, "There was a point when we went, 'What is wrong with this woman?'"

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It was a big moment for Roberts, who revealed she "worships" Gwen.

"I was wearing my Gwen Stefani T-shirt in the car," she said. "You couldn't really see when I put the seat belt on."