It really depends on the standard and how dedicated you are obviously. Retiring age for footballers is mid thirties for positions that require pace and late thirties for goalkeepers and centre backs. However, you can obviously continue playing playing football into your 60's and 70's if you so which, albeit at a lower standard. Hans Rey may still be riding at 43, but he wouldn't be able to stand up against younger riders anymore. Most professional sports find you peaking at in the late to mid twenties, and tailing off after 30, so I imagine it's the same for trials in terms of physical condition. However, dedication plays a larger role in trials, it seems to me the average, non professional, trials rider tends to "retire" in the early twenties due to cars, women, university and work coming into their life, and of course just growing up and becoming more sensible and realising that leaping off a massive wall onto a rail could potentially hurt and isn't worth the risk, something that doesn't seem to cross your mind so much when you're younger. Obviously this won't apply to professional riders who treat trials as the number 1 thing in their life and will probably follow the same curve as the rest of sports professionals.