Michele, I am not sure if the behavior you describe is aggressive biting, play biting or over-grooming behavior. Since you indicate that the older horse is allowing your gelding to bite him, my suspicion is that it is one of the two latter choices. However, my first suggestion is to have the 21-year-old examined by a vet to rule out cognitive changes that would cause him to cease defending himself.
Next, observe the interactions to see if there is a pattern. Does your gelding engage the older horse in play? Does he begin to groom the older horse and then become excited? Does the 21-year-old initiate interactions with your gelding?
It may be that there is little in the way of visual barriers and enrichment in the pasture, and your five-year-old is a horse who engages in oral play—and has little alternative other than this other horse. You may want to try adding hidden treats, Jolly Balls™ and other enrichment items to the pasture to see if the behavior decreases.