At least for me (61-1/2 today) I'm just SLOWER than I was even five years ago. I was able to maintain about the same level of performance until recently--I mean, it varied with my fitness level, but I always was able to get back where I'd been.
I generally gain a few pounds in winter, and I started late this spring, so I'm working at it now. I'm feeling better than I did in May, and I'm confident I'll improve quite a bit by the end of summer. For the first time ever, though, I don't think I'm going to get back to where where I was last September. Dunno if it's because I just don't want to push as hard as I used to, or if it's really age.
It bothered me a little at first, but now I try to focus on the guys my age who couldn't ride the hills at all, not the 30-year-olds who pass me....