I didn't say my body is going downhill. In fact, I'm probably stronger and in better cardiovascular health than I was in my mid-20's. Still a chub, but can do more. Living a sedentary lifestyle is probably the worst thing you can do to yourself. My boss is the same age as my Mom - he runs ultra-marathons and participates in Iron Man competitions (and finishes at the top of his age group much of the time). In fact, most of the people I know that are in to this stuff are well in to their 40's and 50's.
Where the age thing is bothering me most is where I'm at in terms of lifetime accomplishments. I haven't done enough of the stuff I'd planned when I was younger, I've got a lot to get done now. The realization that some of it may never happen sucks.