Well I may be old and out of shape (certainly compared to how I was back in university) but I just keep getting slowly better.
I learned to do a standing tuck in my mid-30's (back in the day guys either stunted or tumbled, very few did both and I was never a gymnast) and started running tumbling at 39. Right now I can do a round off with as many back handsprings as will fit on the diagonal (usually 4), standing 2 handsprings to tuck and can throw a double toe tuck (when my groin pull finally heals) all on a sprung floor of course. My goal is to add a layout to my tumbling in the next year or two and start work on a standing full -- though my tuck has always gone through an up/down cycle and lately it's been down so the standing full is still a ways off.
Stunt wise I'm actually better than ever too. Rewinds, full-ups, tick-tocks... pretty much all the basic and intermediate skills as a base rock solid. I'm not one of the beast guys (I'm only 5' 7" and 185 lbs) so you won't see me throwing double back rewinds or one-arm rewinds any time soon (if ever). Not bad for only one practice a week and the occasional open gym.
I do have to say it does bug me a bit when I hear other people say "I'm too old for this" and it's coming from someone in their mid 20's. This certainly isn't easy for me, I do have to work on it and maintain a healthy lifestyle in general, but if being involved at a competitive level is important for you then it is possible to do it.
I do have some sympathy for people who have been injured and can no longer physically manage the skills; after all, this stuff isn't supposed to be easy, we only make it look that way. But, I've had shoulder issues my whole adult life (sub-luxation -- think of dislocating your shoulder and having it pop back in right away, all the pain and swelling of a full shoulder dislocation without the necessity of seeing a doctor to get it put back in), had ACL surgery on both knees (not cheer related), suffered a neck injury, and had my thumbs and fingers jammed, popped and dislocated more times than I want to remember (darn, now that I'm reminded of it my left pinkie is hurting as I type this -- stupid 'A' key, why do I need you so much?). Yet all of that is relatively minor compared to the guy stunting in a wheelchair!
I'm not saying that everyone should stay in competitive cheer -- there are lots of reasons to get out. For me, I've done the coaching thing, I've been 'retired' for a bunch of years but when it's all said and done I still love to perform so I came back and worked to improve my skills. I know that I'm likely to only give it a few more years from here but I intend to make them my best years yet. Other priorities will eventually pull me away and I'm ok with that, but it won't be because I'm "too old for this".