The average age for girls starting puberty used to be around 12. Now, it can begin at seven years old or earlier. But it hasn't always been this way.
In the mid-19th Century, the age of menarche (the first menstrual cycle) declined in European girls from 17 years of age to 13 - and it's still declining.
Experts believe this is due to increased fatty diets and stress.
Initially the declining age wasn't seen as such a bad thing.
The older age was associated with being undernourished. Meaning, the younger age was "largely due to better nutrition, hygiene and health," writes the Conversation.
The scale has tipped the other way and finding adequate nutrition has been "replaced with a new challenge": obesity. Studies suggest the epidemic could be why girls are entering puberty earlier.
Head of Clinical Obesity Research, Baker IDI Heart and Diabetes Institute John Dixon said this is because fat cells are hormonally active.
"The more fat cells you have the greater tendencies there are for estrogen like chemicals to develop early, which may be part of the reason we are seeing earlier puberty as the obesity epidemic increases," said Dixon.
A child starting puberty at a young age, being forced to grow up before they are ready can be very damaging mentally and physically.
"From depression to eating disorders to all the problems that come from being viewed as a sex object while still a little girl," reports Fusion.
"But on top of the psychological and social challenges, getting your period young is associated with a slew of health risks. Notably, research suggests that women who go through menarche early are more likely to develop breast cancer, diabetes, and cardiovascular disease. All of these correlations are currently being explored," writes Taryn Hillan.
However, weight gain does not explain everything.
"First of all we don't understand why puberty is earlier in the community generally but we equally don't understand what is driving this obesity epidemic," said Dixon.
"And it is quite likely that there are common elements to the interaction between the environment and the condition that's occurring."
There is a causal link between early periods and obesity because of the hormones but Dixon says, " these two problems could be driven by the same issues in our environment which we have not sorted out."
"Early puberty in girls is telling us there's something wrong but we need to actually look more openly and intelligently at what's going on."