I would assume it depends on the diet and condition of the stallion. Age does have an effect on fertility, but diet, and not just body condition, but mental condition could have its effect as well. I know of some older stallions that run with a few mares and get the job done. It may not be efficient for hand breeding, but if all he needed was to breed a few younger, more fertile mares I would think it possible. But I would not doubt a stud in his 20s would be less fertile than one in his prime, but they would not necessarily all be sterile.
I could believe a breeder wanting to let go of an older stud, especially if they are getting out of breeding. It just breaks some people's heart to get their stallion cut after years of use, (regardless of the quality of stallion,) so sometimes you'll see them sold to breeders that are just starting up and need a seasoned stud. Breeds that are more prone to be high strung or show horses that don't get to interact with other horses might have high stress and therefore are less fertile. Only way to really see if they are fertile is collect and get a sperm count. My neighbor had a younger stallion that seemed to be shooting blanks so they got a count to see how he was producing. I figure you just send it in to a certain lab and they look at it for a fee. Hope this helps a bit.
Last edited by ButtInTheDirt; 03-03-2013 at 05:32 PM.