Former Alias star Jennifer Garner is still kicking butt at age 40!The busy mom, who has three kids with hubby Ben Affleck, looks amazing for her age -- which is why David Lettermancouldn't believe she had her milestone birthday in April. "I like being 40," she told him. "I really like it. I had no problem with it. I feel like, what more could I want? I can't speak for [my husband], but it doesn't look like he's as happy as I am about turning 40." This year she starred in Butter and The Odd Life of Timothy Green, plus gave birth to her sonSamuel. Next up, she's filming The Dallas Buyer's Club, and she shows no signs of slowing down.