Getting older is such a gradual thing that it sort of sneaks up on you, IMO. Or I should say, in my
experience. I am 47 and was blessed w/ pretty good genetics health-wise, frankly I have never had to exercise much to stay in decent shape. My Dad abused the living daylights out of his body by smoking Camel straights and drinking rivers of scotch, and he always looked like a million bucks and played a mean game of tennis a couple times a week until cancer finally struck @ 70. He is still alive @ 74, he beat the cancer for the time being.
That said, i have reached the age where I definitely need to exercise and eat right just to stay somewhat even w/ father time, (ultimately a losing battle)

, and like Dan alluded to I think that it is important to maintain hobbies and interests that you truly enjoy, (sports cars/racing/etc.), but never because of some "mid-life crisis". I cannot imagine being a slave to what other people think of me at this age, it is just so unimportant. Trying to impress women, or whatever. Of course I felt the same @ 25, so that's just me.
The best part of aging is knowing one's self, and what makes you happy. (And sad).