I don't think it's a requirement for you to be 18 to apply for universities, but you will definitely need at least a high school diploma. Most first year students over here tend to be slightly older than freshmen in the US (18 and 19, or older), so if you aren't 18 yet, you'll probably be quite a bit younger than most of your classmates.... unless you go for a foundation year. Are you applying directly out of high school?