Just find somebody that thinks s/he is faster than you, and go for it.
If you want to get into organized racing, the 80 classes normally do cut aff at 15. The qualifying age is typically determined on January 1, for the entire year. This means that if you were 15 on January 1, 2006, you should have no problem racing in the 80 beginner, 80 12-15, Supermini (if you think you're fast enough), or whatever 80cc class your local track offers. For more information, contact your local track or race promoter, they actually determine if you can race, and what class you belong in.