There are many changes that occur to a female body during puberty, primarily because of the action of the hormones estrogen and progesterone. On top of this, hair and skin may become oily, and you may experience cramps, mood swings, irritability and fatigue. Go here for an article on the timing and stages of puberty in girls. Amazingly, all these changes are preparing your body physically to be able to conceive and bear children. However, it can take a lot longer to be able to cope emotionally, mentally and spiritually with sexuality and having children.
Menstruation is the major biological process which facilitates reproduction.
That is, when a girl starts to have a regular menstrual cycle it means that she is fertile and is able to have a baby. On an emotional level having a regular menstrual cycle opens a young woman to her intuition and instincts, and the world of cycles. Nothing is ever straight down the line, or clear cut anymore. A young woman becomes aware of the shades of grey. She may start to sense more vividly the hidden motives, thoughts and actions of others.