Consider that what you learn between 5 and 15 is nothing to what you learn between 15 and 17, simply because in that time you develop the maturity and complexity of understanding to actually APPRECIATE what you're doing with music.
I started playing the clarinet just before I turned 16, and in three years I went from total n00b to around grade 5 AMEB. That was six odd months ago, when I stopped playing to concentrate on the HSC etc. I haven't really picked up my [ebony] stick since, partly because I never got around to buying my own.
The reason I developed so quickly was because I went clarinet mad and practiced about two hours a day for ages, even when I first began. I'd have the clarinet next to the keyboard (computer) and would play until I got tired, answer people on ICQ for a bit, and start playing again. Worked like a charm.