At your age, open source is the best way to learn. That way other developer will be able to point out some of the mistakes. And point you out on the right way to do something.

Besides this is not something that any developer cant do in a couple of hours of work. So not releasing your code doesn't really have any impact.

But 15 years old and coding is impressive. I was rotting day and night playing Civ 3 at that age :) Wish i didn't :)