Unhealthy lifestyles have reduced the onset of puberty age by more than five years in girls, claim experts.
Some girls are only six or seven years old when they have to cope with their first period, the Sun reported.
A US study has found that boys are also hitting physical maturity two years earlier than they did in the past.
Scientists found that the average age for the onset of puberty in girls was 16.6 years in 1860.
But by 1920 it was 14.6, in 1950, 13.1, in 1980, 12.5 and in 2010 it had decreased to 10.5.
Experts fear that this problem could lead to an increase in young girls getting pregnant.
Marcia Herman-Giddens, from the University of North Carolina where the latest research was carried out, claimed that obesity may well to be blame as it alters the body's hormones - with some starting to show the first signs of maturity as young as six.
The average age for white and Hispanic boys is 10, while it is 9 for black boys.
Tam Fry, honorary chairman of the UK's Child Growth Foundation, has explained this as a "ticking timebomb" for today's society.