Being 13 isn't in itself prohibitive when it comes to racing, There are several cases of horses still winning even beyond the age of 13. I would liken it in some ways to a car. It depends how many miles are on the clock, how well they have been looked after and whether or not they were run in considerately. But don't take me too literally on that. Obviously age does creep up and degenerative arthritis might be starting to creep in. The main problem I suppose is that the horse may have the mentality to want to race and it may well have the ability but, as with humans, the after effects of the exertion and the recovery rate are not as they used to be (as I discovered in a boxing match on XBox)