A: No, you can wear a black hat at any age. However, it seems to have become the custom for boys to start at Bar-Mitzvah, and you should be sensitive to the sort of reaction you might cause, if you decided to be different. If you have a good reason, by all means, go ahead. I myself started high school a month before my Bar-Mitzvah, and as everyone else in the Yeshiva was already wearing a hat, I started wearing one then as well. Use good judgement.