Training and dog selection are the integral part of race strategy. Either select dogs to fit your strategy or select your strategy based on your dog's strength. In any case, train your dogs to do what you expect. Don't cheat your dogs. On the other hand, I do not ask more of a dog than he or she has to give.
Older dogs love it the most.
Running is so completely a part of a sled dog's identity and dignity, that it seems unfair or even cruel to not run an older dog. What an older dog may have lost in speed is made up for by the trail wisdom to pace expension of energy and focus on the task. They are great teachers of young dogs and are a source of great joy and satisfaction to any dog musher.