Specifically, physical / mental changes to your body versus stuff like not knowing who a current celebrity is.
The change that has most affected me is the need for reading glasses. I'm apparently somewhat lucky in that I didn't have the need until after I turned 50. A lot of people I know started needing them after 40. It bugs the hell out of me, more, I think, than if I were to need a prescription for normal vision. If that were the case, I'd just have the glasses on all the time, or would elect for laser surgery.
With the stupid reading glasses though, I end up keeping like a dozen pairs in everything from my toolbox to my fly fishing vest to my vehicles, and then I still end up sitting in a dark restaurant or going to the Home Despot and not being able to read fine print because I don't have glasses on me because I usually don't wear the kind of clothes with a pocket that will hold them comfortably. It's just super irritating not being able to see small stuff on my own without help.
A (currently) more minor thing is forgetfulness. I find I have to, more and more, leave myself notes about everything, and on the more serious front, have occasionally forgotten to turn a burner off on the stove or similar.
When I was younger and my hair first started thinning, I thought that would be the worst "old guy" thing. That hasn't really bothered me at all though. I've embraced the buzz cut and prefer it to long hair.
So what makes you feel old?