From the Globe and Mail:
A recent study published in the journal Pediatrics, looked at 1,239 girls aged 6 to 8. By age 7, 15% have started developing breasts; by age 8, that number increases to 27%. Those numbers are alarming because they're significantly higher than numbers reported in a landmark study from 1997, which showed that only 5% of white girls had those signs of puberty at age 7.
Additionally, overweight or obese girls are far more likely to grow breasts at an early age. Race is another contributing factor: The study found that 23% of black girls begin puberty at age 7, compared to 15% of Hispanic girls and 10% of white girls.
The danger of girls maturing physically at such a young age is that they may look like teenagers - and may be treated like sexually mature young women while they are actually still children. The authors of the study also mention other dangers associated with early onset of puberty: “Earlier maturation in girls is associated with lower self-esteem and less favourable body image, as well as greater rates of eating problems, depression and suicide attempts. They were more likely to be influenced by deviant peers, with earlier onset of sexual intercourse.”