In nature, as for any living organism, the probable sole aim of the human body is to reach maturity and reproduce before it dies, in order for the species to continue. Girls are born with all the eggs they'll ever have; in fact, the number of viable eggs is constantly falling. However, it takes on average 12 years for the female human to develop to puberty and fertility.
It's not clear why it takes this long, although there's probably a critical weight - around seven and a half stone - at which menstruation is triggered, dependent on the other hormone systems functioning properly, including growth hormone.
The changes puberty brings are vast, prompted by rapidly increasing levels of many hormones including oestrogen, progesterone and androgens.