Answer:
Arm-pit hair, along with anything else in puberty, varies from person to person. You could not get it until you're 18, that's not likely though. If you have pubic hair, then that mean you have started puberty. The next thing is darker hair, and longer hair on your body, voice change, wet dreams..etc. Those are all signs of puberty.
Anyways; your best bet is probably to just let your body progress, and not worry about it.