It's pretty much just genetics and a bit of self maintenance. Tom Cruise certainly knows that his youthful and healthy looks are part of his value as an attractive leading-man type. He's fortunate that he's got good genetics to assist in that. As for the costar actress? She doesn't seem to care much about her appearance (as it pertains to a feminine, 'pretty' look). She's aged quite a bit (a normal process) and seems to have given in and accepted it.
I guess that's another reason we are all different.