Girls are now hitting puberty at as early as five years old, according to experts who also found that the average age of puberty onset in girls has dropped by five years in the last century.

Recent research shows that on average girls start experiencing puberty at the age of 10, according to experts at UK's Cheltenham Science Festival.

Tabitha Randell of Nottingham University Hospitals NHS Trust Thursday that she has even seen a girl as young as four developing body hair and body odor.

Experts believe that the early onset of puberty is caused by a hormone imbalance, which can be caused by a number of factors like the rising levels of obesity or the stress of family breakdown.

Experts say that when a child's fat tissue level reaches a certain level, a hormone signal is promptly sent to the brain triggering puberty.

"You get these occasional children who have a genetic mutation that means their bodies can't send that signal. So they continue to eat and eat, but don't go into puberty," he explains. "But they're the exception," Professor Richard Sharpe, an expert in early puberty at the Medical Research Council, said at the festival, according to Huffington Post.

Sharpe said that early puberty can also be caused by the higher expectations put on girls to achieve at school and fit in with the crowd.

In 2010, Danish researchers found that breast development now begins on average at the age of nine years and ten months, a year earlier than two decades ago.

"If girls mature early, they run into teenage problems at an early age and they're more prone to diseases later on," lead author Dr. Anders Juul, of the Copenhagen University Hospital in Denmark, told Parentdish at the time.

"We should be worried about this, regardless of what we think the underlying reasons might be," Juul said.  "It's a clear sign that something is affecting our children; whether it's junk food, environmental chemicals or lack of physical activity."

Not only does early puberty increase the risk of breast cancer in later life, it also risks premature sexualization of young girls and teenage pregnancy.

"We are seeing early puberty in situations which we would deem stressful for girls, for example being adopted or growing up without their biological father in the house," Sharpe said, according to the Daily Mail.

"There may be a stepfather or the child is having to live in two homes. That is all somewhat stressful and it is much more common nowadays than it was," he added. "It may even be that it is all the expectations of the modern world on girls, because there’s been a radical change in what their expectations are."

Previously, US researchers found that girls whose father had left home were 2.4 time more likely to develop breasts at a young age. 

Researchers at the University of California Berkeley studied 440 girls who didn't have father figures and found that on average these girls began puberty at the age of nine and some as young as seven.

"We need to help parents deal with the physical and psychological consequences of early puberty which are very serious and can very distressing for the child," Randell said.