There isn't a certain age. It's basically whenever your boobs start looking noticable and when they need support. It's different for different people. Some are early, some are late. Whenever you GET any boobs I guess is when you should start wearing a bra.
when you develop boobs, but if you havent develpoed when you get into 6th grade you should wear one because you wouldnt want anyone to see your chest even if theres nothing there. Start with a training bra.