I am turning 40 in two weeks. I feel like a teenager mostly because I've stayed in great shape by running and light weight-lifting non-stop for over 27 years. I
see young people's eyes go wide when they hear my age, but admittedly, I do have some "old" moments.
Aging is really nice when you stay in really good shape. The trick is to train with younger guys and hang out socially with older people (or even my own age), I find. It makes you feel ageless. The young runners make you forget completely your age and then, when you engage real world people, you are the youngest looking guy in the room and everyone reinforces it passing you some envy and nice commentary.
Plus, every woman over forty looking like Jennifer Aniston will bat eyes at you.