Im 17, and I have my horse boarded at a stable. I drive myself, take care of my horse myself, and very rarely is their anyone out their to "watch" me. When I was 13 I was being dropped off at stables to ride alone as well. I think as long as the kid isn't stupid, knows horses and how to respect others and their animals, and the parents aren't stupid, and sign a waver it shouldn't be a big deal. I've never had problems anywhere.