I wouldn't get a Thoroughbred at 15, especially not a young one.
I got my first youngster [an OTT standardbred, broke but very green] when I was 14 and it was a DISASTER. Every teenager goes through a reactive phase and reactive is THE LAST thing you EVER want to be with a young horse.
Over the next two or three years I learned a lot. I got an 8 year old Welsh pony, and he was a naughty little sh!t but I learned a lot from him. A year after getting him I started riding my young Standardbred again and things went MUCH better. Unfortunately my Standie ended up being retired and then euthed, and the horse I got next, when I was 16, was a 15 year old Anglo Arabian eventing schoolmaster.
I LEARNED SO MUCH FROM HIM. I can't even tell you how much I learned. In about June or July of that year I got my first foal, and the ONLY reason that worked out was because of Monty. When that foal was nearly 2, it became painfully obvious she wasn't going to get tall enough for me, so I gave her to my mother and I got Magic, who is a couple of months older than said foal.
I was nearly 18 at the time. And Magic was dangerous. I did NOT have the experience to handle her and by all rights should not be sitting here typing this message! She should have sent me to hospital many times. Not once have I ever been hurt because of her. I have gone to hospital from a fall off her once, but that was 110% my fault.
Now, I am nearly 20, Magic is nearly 4, and we are both entirely different people to who we were two years ago. Magic is a quiet, reliable member of equine society [BUT I MUST STRESS SHE HAS NEVER SEEN THE TRACK - I got her unbroke, she never even made the breaker before she was thrown away] and I know so, so much more than I did then. I have been asked to break two horses to saddle and I'm very keen to get back to riding from this darn knee injury [not horse related]!