A castration can be performed on horses as young as three months of age and is normally done on young horses before they reach 1 year of age. Gelding a 10 year old stallion is not a common practice and needs to be done with more care and attention. More specialized surgery is required in this case due to more bleeding. Also the recovery time for an older horse is longer than for a young one. You should always consult an experienced equine veterinarian for this kind of surgeries that carry a greater risk for your horse.