I’m still a teenager, but I’m “adult sized”. At 5’4″, I still ride, show, and help train ponies. One of my mares is still considered a pony at 14.1, and I ride her consistently. Last summer, I rode my sisters old 11-12hh spunky pony, and during the winter I rode a quarter pony about the same height and a Kentucky Mountain Saddle Horse that’s around 12-13hh. None of them have had any trouble carrying me. It’s acrually pretty common in the gaited horse world to see fairly large men riding ponies who’s legs are about as long as the man’s. Icelandic Horses are usually pony sized, yet grown adults ride them all the time and they don’t seem to have any problems just riding right along even at faster paces.