I'm definitely no expert on any of this, but I do know even if you have trained some horses, if you get the wrong young one it can really hurt you. What if you got a 7 year old very talented jumper, then you can move it up the jumping levels or something, maybe learn how to event on it and stuff. You don't need to get a young horse just to get a challenge, get an 10 year old and learn how to event on it, or something.
I am currently teaching myself how to stand up on my horse while she's cantering, sometimes the challenge doesn't have to be the horse, try working hard on dressage and your seat and timing and everything in jumping?
Just my 2cents if your parents don't want you to get a super young horse.
If there are no horses in heaven... im not going.