Personally I don't think age really matters (within reason), but I think it really is personal preference. Obviously depending on health and general fitness, but over all I think someone could quite easily ride trials and enjoy it in their 30's. I'm only 17 and when I did ride trials I rode for about 5 years, then I quite for about 1 month, felt lost without it so went back and I have to say, trials is such a great, enjoyable way of keeping in shape.
As has been said, I don't think age should bother you if you're up for trying something new... trials is a great sport, it has plenty of variety, great social side and as I've said, keeps you fit while you have fun. Best of luck with taking trials up as a hobby though! I'm sure you'll enjoy it.
Shep.