Taking my comments with a pinch of salt as I’m just beginning my writing experience as well, I’d agree that any kind of writing is good experience.
I’ve tried both novels and shorts, and I’ve found shorts keep my attention better because I can rattle them off quicker – although they need as careful attention to editing and redrafting as a novel – and give a good sense of achievement when you’ve completed them. Concentrating on them has rapidly given me an ever-growing body of work I can submit which gives me a kick when something good happens with one of them.
I believe shorts take a different kind of writing to novels, so if you want to write novels, you’ll need to practise that skill as well. Both are good fun though.
As regards writing a novel at your age, I believe there are many successful writers who started early, Christopher Paolini of Eragon fame being not the least.
I write Science Fiction myself, and several of the magazines I submit to publish Fantasy as well. If you want to drop me an email, I can pass on the addresses I’ve got if that’s any use?
Cheers,
Smurf