I have never seen anyone start out that late and turn Pro in Motocross. Bob "Hurricane" Hannah didn't do his first race until he was 18 and look at the legend that he became in Motocross. I do know of a guy that started out in Enduros at 29 and became what is the equivalent to Pro in that sport.
Yeah just head out to the Track way early. Sign up for a Beginner Class Race, buy your Day Pass or a Years License that day of the Race, bring your Riding Gear, Extra Pre-Mix, Extra Money and A Friend (incase you wreck so bad they need to drive you home. I am being serious), and DO IT! Go for it. I can't wait to hear how you do.