Girls start to grow into women and boys into men. This process can take a year or even six years. This time period is when a child's body takes on those characteristics that define it sexually. Keep in mind that everyone matures at their own pace, but eventually we all catch up. Puberty leads to adolescence.
Adolescence can be a very difficult time.
You are no longer a child, but you are not an adult yet either. Hormones is the answer. Your brain knows when your body is nearing sexual maturity. So it releases chemicals called hormones. Different hormones are responsible for different changes.