You will know when you have hit puberty when you have irrational mood swings, if your a girl your breasts will enlarge and form and your period will start (some people later than others), your hips...
Some signs are you are growing boobs, growing hair on your vagina, armpit hair, you might get taller, have a fast growth spurt, and receiving your period.
For a young girl around the age of nine you'll feel your body start to change. The most noticeable changes at first will be that your nipples will swell a bit and your breasts will begin to 'bud'....