I doubt this really needs to be said, but it's not age. It's wisdom, which is basically just experience. The more stuff you see pass away, the less enchanted you are with it. You know that you will eventually be separated from things, so it gets easier when it happens.
I'm trying to think of really old, selfish people. Not super easy. And it's pretty hard in every case not to see those individuals' younger selves as being more energetically, vigorously selfish. :-) Ahh, to be young again...