Here's how it will happen if you start up now. Your first start-up will be an unmitigated disaster. Your second one will be slightly less so, but still fail. Your third will break even. Your fourth will finally make you a reasonable sum of money. Then your fifth will make you rich. Allow for 1-3 years per start-up - with this, you'll probably be very wealthy by the time you're 27. However, you'll also have wasted the best years of your life working your ass off.
Wait a little, and chances are you might even be able to skip one of those failed start-up - after all, life experience does count for something. Finish school. Go to a cool university. Learn lots of interesting stuff. Meet lots of really interesting people (that you'll be able to recruit into your future ventures, among other things - but more importantly, some of them will be your best friends for the rest of your life - money cannot buy that). Sleep with lots of girls. Get drunk. Enjoy life. Wear sun-screen. Erm. Anyway, you can always start up after uni. If anything, it will be easier, since a) you'll be wiser to the ways of the world; at the moment your inexperience pretty much guarantees dismal failure, b) maybe we won't be in the middle of a credit crunch then.