The mean age for a girl's first period is 12.75 years. Wide variations are seen in the sequence and timing of these events, but peak growth velocity always precedes menarche. Precocious puberty is defined as the onset of true puberty before 8 years of age in girls or 9 years of age in boys. Precocious puberty is 10 times more common in girls than in boys. Sexual development may begin at any age.
Most precocious puberty is simply early maturation.
When puberty occurs before 6 years of age, it is important to look for other causes such as ovarian cysts, thyroid problems, McCune-Albright syndrome , or external sources of estrogen. In girls over age 6, these other causes are quite rare, but should at least be considered by your pediatrician. Your friends have suggested that bovine growth hormone is the cause of your daughter's precocious puberty. I strongly doubt that. The only external compounds clearly implicated in girls' early puberty are estrogens.