My youngest horse is 15, and I consider her young, my oldest is 42ish, he was my first horse, I got him at the age of 28 and rode him consistently until I got pregnant and stopped riding(6 years ago). My point is, I don't consider a horse old until they get to about 40 lol. Blue(my old guy) could still be ridden if it weren't for a injury that happened a little over a year ago, and it would take a while to build his muscle back up. Other then the slight hitch in his gait from that injury he would be completely sound and willing to work(and still crazy enough to gallop me home the second I let my guard down on the trail). My finial horse is in his early 20's and is also completely sound and a very easy keeper.
In closing, I think 15 is a great age, actually if anything I consider them a bit young