How can you know when you start and stop puberty?

Body changes: Girls start breast budding as the first sign of puberty; for boys it's an increase in testicular volume. Pubic hair is associated with puberty, but doesn't define it. Puberty ends when growth is pretty much done and breast contour is adult or testicular volume approximates adult size. Pubic hair span usually extends to the groin, but doesn't have to by the end of puberty.