I know it sounds like a bs answer, cliche, etc., etc., but it really doesn't matter. You're aging, I'm aging, we're all aging, it's just happening. The only thing that would make one feel "old" is how you compare to another person's aging process. To compare yourself to your younger self is sort of silly, because it's not as if you're wishing you'd done something differently. It's not as if you can truly regret the aging process.
We all do it differently, too. I'm faster now than I was at any point in my life, but this is because I did the classic "smoke and drink and damn near hill myself in my teens and 20's" bs routine. I just turned 40 and I'm not trying to sound like a Hair Club for Men commercial when I say I've never felt better.
Will I run this fast when I'm 60? Duh, no. Do I care? Not really, as I'm just going to continue to do whatever it is I can do to keep life good.
Unless you really wreck something one day, the "decline" is so gradual, that's it's hardly worth noting unless you're constantly regretting your life or wishing you'd fashioned it like someone else's, in which case, you're already old (and you're screwed.)