If you like it, if it fits your face and overall style--go for it! Just take the time to groom it into a style. As noted above, there's a difference between growing a 'stache (or beard/goatee/etc) and just not shaving.
I've been able to grow a full mustache since I was 15 or so and started shaving at the age of 12. If its thick enough and full enough then go for it. That said I never really sported a mustache in my teens, even tho I did grow it out a couple times.