It depends on the horse and what age they were started (if they were started too young/ridden to hard), if they have had proper warmup time/cool down time, etc. You never know what's happened in those 15 years but your best bet is to get a PPE done.
I am buying a 15 year old pleasure mare tomorrow. I plan on her teaching me quite a bit over the next few years. Totally depends on the horse, like someone else said...some can be sound and fine into their 20s...others become lame young. It just depends. Get a pre-purchase exam. Our 27 year old mare was ridden right up until she was 25, now only ridden lightly in the summer and that's because no one took good care of her throughout her life...rode too hard, too young and when she started to have joint problems they dumped her. She's still a spunky old lady though!