I agree with what everyone has said. :) And I thoroughly second smrobs.
My mare was 23 when I got her (although, the owners lied/forgot her real age and told us she was 21 then we saw her vet records and discovered that she was 23) and if you just looked at her (without knowing her age), you would have absolutely no clue that she's 25.
She was horrifically obese and super out of shape but I got her back in shape and now, at 25, there really isn't anything I couldn't do with her. Of course, there are things I don't do because I'm trying to keep her well preserved, but she thinks nothing of 4 hour trail rides, daily medium work, galloping, cantering for long periods, etc. She doesn't have any obvious arthritis, but she's kinda special like that, and she definitely has at least 3 more years of hard work usefullness left in her. Inside, I feel like I could say 5 but I don't want to be tooooo optimistic. In any case, I'm pretty positive that she could have up to ten years left of pony rides and such.
I really think older horses are HIGHLY underestimated. If he passes the PPE and you guys/your kids and him click, go for it! You never know, you could find yourself with a real gem on your hands.
Fabio - 13 year old Arab/QH gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.