What a silly question, of course you can still be a good racer. Many good racers started at that age. You don't have to start racing while you're still in diapers to be competitive. One of the top offroad riders in the world, Juha Salminen, didn't start racing until late in his life. The guy was such a good rider that his friends pressured him into entering races. He's won numerous titles including GNCC, W.E.C, Erszburg Rodeo and others. So no, I don't think age plays much of a difference, maybe when you get older and you can't bounce back so quick from an injury, but not now LOL. You never mentioned whether you had any kind of seat time on a dirtbike yet. If you don't, you'll need to start off with something predictable and manageable. Look at something like an air cooled trail bike just to get familiar with the basics. Once you have all the controls and body positioning down, you can always upgrade to a mini or supermini 2 stroke or 4 stroke.