I too have worked around horses most of my life, it seems that teenage girls like to think of horses as friends and pets, they are neither.
Me and SWMBO recently got a Belgian Warmblood from a horse barn that catered to young riders. This guy is 17.4 hands and 3/4's ton of bad attitude, an animal like this requires constant vigilance when handling, he will hurt you.(but he can also jump a 4 ft. fence without blinking) I can't imagine a 17 yr old of either gender being able to handle this horse.
Three day eventers "have to own a crazy horse" it's in the rules.
Disclaimer: I no longer ride, I consider myself too old and fragile.
basilisk