A yearling stud could easily get a mare pregnant, especially if he is dropped and mounting mares [and geldings]. I know a person who has a stud colt out with his dam, and he mounts her and the gelding he is with. I have no doubt that he'll get his dam pregnant after a while if nothing changes. But some people don't even think of this as a problem. (Because he is with his dam they don't think he'd try, but have seen him do it.) I am so glad to have a filly, because frankly I think the whole gelding situation can kind of be a pain in the butt. Certainly something that needs to be done, though, but I think some people just delay it because of the old wives tales that somehow, leaving them with their bits is better. (They can't get mares pregnant at that age, they'll grow bigger, etc.) My gelding was gelded as a long yearling, and he even acts like a stud. No mounting mares or the like, but just his demeanor is different than the average gelding.