This is a very personal decision and only you really know your dog best. Having said that though, there are several products that can help your dog if he is stiff and in pain from his aging joints. Your vet can prescribe pain meds and/or you can even try some supplements. Is your dog on any glucosamine? Both my dogs got pretty good mobility in their last years using YouMove by Lintbells. There are many great products. If your dog is losing weight, consult with your vet to determine why. Cancer is sadly rampant in older dogs so you need to find out if there is something else going on. While it is true that old dogs lose muscle mass and some may become finicky eaters, losing quite a few pounds deserves investigation.