Each horse should be judged on an individual basis. Use your best judgement and if you increase his workload slowly you may end up with a sound, inshape, eager to go riding partner. When you have your vet/farrier/trainer out ask them to give him the once over and an opinion. Remember ask 5 people get 7 different answers. Let the horse tell you how much is to much. My daughters pony who she rides daily religously over the summer is 38. Just got a new horse in age 7 who had such poor care and mismanagement will probably take 6 months before sound enough to start riding. Age is more than a #.