Look into feeding a higher protein feed, give them a really good warm up and cool down for every single ride and as Niamh says, massages and stretches and breaks when they need it
I have a 17 year old Warmblood who is in amazing shape. We compete in A and AA rated shows 2 monthly so he is constantly being worked. My horse gets electrotheropy twice a week to help with soreness. Personally I think it really helps to keep him in great shape!
I ride a 20 year old horse and what I do is massages and warm up exercises before training and give your horse breaks to get its energy back