I believe that as long as a horse is comfortable being worked, then he should be worked. It is movement that keeps those joints lubricated and often it is when a horse is retired and turned out to pasture where he walks from feed to water then stands around the rest of the time that they really start to go downhill. I don't have much experience with senior drafts but have been around my share of senior QHs. Just make sure that he is on a well balanced diet. Solon knows of a book that is like a Bible for draft owners, you might PM her about that. Other than the joint supplement, so long as he is keeping his weight, I really don't think there is much else to do. Of course, watch for signs of colic and founder like you would with any other horse of any age. With as good of shape as he seems to be in, he should be up for almost anything. And if the Amish trained him to drive, then he should be VERY broke. At least all the horses trained by the Amish in my part of the country are.