I started just before I turned 16, and there are some very famous magicians out there who started around that age too, so don't worry. Besides, what ultimately determines your success is not when you started - it's how much work you put into it. Tortoise and the hare

I agree with everyone else, as you're just starting off, get books. When you visit magic stores, online or in your city, you'll be enticed by single tricks that are popular and well reviewed, but that money is much better spent getting a good foundation first. Every area of magic has a couple of "bibles", books that have been widely accepted as the best way to get start off in that type of magic - you can find those after only a bit of research, and they're definitely a good choice.
If your library doesn't have any magic books, I recommend searching online libraries. I discovered them recently and they've been my secret weapon.
As for how much practice you need before you can perform in front of people, that's completely up to you. Just make sure to only perform tricks you're comfortable with, and don't need to focus on remembering the procedure - that way you can put most of your attention on the presentation.