Matt Damon "can't wait" to turn 40 -- and enter the "prime of his life," as his "Invictus" co-star Morgan Freeman puts it.

"Morgan said, 'Oh, you're going to be 40 next year' and I said, 'Yeah,' and he said, 'Prime of your life,'" Damon tells "Extra."

"This is the best time of life, between 40 and 60," Freeman, 72, says. Matt chimes in, "I can't wait."

One person who may not agree with Freeman's assertion? "Invictus" director Clint Eastwood, 79.

"I say that, but Clint's pushing what, 80?" Morgan says. "It's his world, we all just live in it," Damon agrees, laughing.

"Invictus" tells the true story about Nelson Mendela's life after prison -- and Freeman has never been more nervous about a performance."

"You're playing Nelson Mandela," Morgan explains of his daunting role, adding that he worried about "getting it right."

"Invictus" opens December 11.