Pubey does NOT occur before the age of 8 years because:

Pubey and regulation of its onset During childhood the hypothalamus does not secrete significant amount of GnRH because during childhood, the slightest secretion of any sex steroid hormones exes a strong inhibitory effect on hypothalamic secretion of GnRH. But at the time of pubey, the secretion of hypothalamic GnRH breaks through the childhood inhibition and adult sexual life begins.