Some of it is genes, as Ogul says. Not every man can build their body the way Ronnie Coleman or Jay Cutler can, steroids or not, for example. But when you're a star performer in an image-based business like Hollywood, you're essentially guaranteed the best of everything: Doctors, trainers, nutrition, and as much vacation time as needed. Your job is to look good, and your bosses will make sure you have every advantage. And when age starts to creep in, you also the best surgeons. It also helps that most of the people we're listing here are just naturally good-looking, which can soften the blows of age.
And then there's our own tunnel vision when it comes to celebrities: We see them as the iconic characters they created, not necessarily as themselves. I think most people perform a slight mental edit when looking at their idols throughout the years, and we tend to see them as they imprinted on us. Tom cruise will probably still be told "Man, you haven't aged a day" when he's 70, partly because of his good genes, partly because of his access to the best professionals, but also partly because to us he's not Thomas Mapother, he's TOM CRUISE. He's Maverick, Lestat, and Jerry McGuire, and that shades how we see him.