Girls are experiencing signs of puberty as early as five years old, and the average age of puberty onset in girls has dropped by five years in the last century.

Research on the worrying hormonal trend shows that the average age girls start experiencing puberty is now 10, The London Times reported.

Tabitha Randell, a consultant pediatrician at Nottingham Children's Hospital, said she has witnessed a case where a four-year-old girl was developing body hair and body odor.

Research conducted on thousands of UK girls in 2010 found that the group was "maturing significantly earlier" than girls in previous studies.

Parents and scientists are unsure about the possible causes. Obesity, however, is the biggest suspect.

Stress is also considered to be an accelerator. US research has argued that prolonged stress caused by, for example family breakdown, could alter hormones in the body and speed up the process of puberty.

"The simple truth is we don't know why it is happening," says Professor Richard Sharpe, an expert in early puberty at the UK Medical Research Council.

"Obesity is the biggest factor that we know of. [But] there's clearly something else. Is it environmental chemicals, is it societal stress? I would say on the evidence, environmental factors are not a major player."

The medical community has agreed that more research is needed, but have advised parents to watch their children's weight.