The onset of puberty has fallen by more than five years in girls and experts believe junk food is to be blamed for this ‘ticking timebomb’.

Some girls are only six or seven when they have to cope with their first period and a recent US study has found that boys are also hitting physical maturity up to two years earlier than in the past, The Sun reported.

Scientists have also found that the average age for the onset of puberty in girls in 1860 was 16.6 years.

By 1920 it was 14.6, in 1950, 13.1, in 1980, 12.5 and in 2010 it had dropped to 10.5.

Experts fear this could lead to an increase in young girls getting pregnant.

Puberty is the process of body changes from the first signs of adolescence to full adult development.

It is brought about by a series of chemical signals, which are sent from the brain to the pituitary gland in the head, then from the pituitary to the sex glands, the ovaries in girls and the testicles in boys.

These in turn make the sex hormones — oestrogen in girls and testosterone in boys, which cause the physical changes of puberty.

Normal puberty takes from four to five years in both sexes but it starts earlier in girls than in boys.

Marcia Herman-Giddens, from the University of North Carolina where the latest research was carried out, claims 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 ten, while it is nine for black boys.

Tam Fry, honorary chairman of the UK’s Child Growth Foundation, has described this as a “ticking timebomb” for today’s society.

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