Though this used to fall under precocious puberty - a term for when girls experience puberty before 8 years old and boys experience it before 9 - it's actually become something of a normality for children.
"In general, we think that 7 is now probably a normal age to have some signs of puberty," Louise Greenspan, a pediatric endocrinologist at Kaiser Permanente who researches puberty, told WSJ. "So the cutoff for precocious puberty is a gray zone now."
It's often something seen among young girls. In fact, a study from 2013 that looked at more than 1,200 girls in the United States found that the earliest signs of puberty, like breast development, were occurring around age 9 - a year earlier than the expected average, according to WSJ.
For girls, puberty can take four years. Breast development may begin at 8 or 9, while the "onset of the menstrual cycle, called menarche," starts around 12 years old, WSJ reported.
Dr. Biro told WSJ girls experience puberty earlier when they have a higher body mass index, since body fat "releases the hormone estrogen, which is released from the ovaries during puberty, causing the start of breast development," WSJ reported.
To find this, researchers followed girls from 2005 to 2012, annually checking the girls' height, weight and health. They found girls whose mothers were overweight during pregnancy were 2.5 times more likely to see pubic hair grow earlier than girls whose mothers weren't overweight during pregnancy.
And developing early isn't always a good thing for youngsters. The new study indicated that those who experience puberty early end up having an increased "risk of adverse health outcomes, including obesity, type 2 diabetes, polycystic ovarian syndrome, and cancer in adolescence and adulthood," according to a press release on the study.
In fact, WebMD also explained that early puberty - for both boys and girls - can create emotional and physical issues later in life. For example, kids can develop a shorter stature, since they stop growing once puberty stops, and an earlier start time means a quicker stop time.
Children who experience early puberty can also have behavioral issues, including desire for sexual activity from an early age.
It also may cause stress, as puberty often does among children in general.
"Even when it's happening to average 12-year-olds, puberty can be a confusing time. It can be all the more stressful for younger kids with early puberty," according to WebMD. "They might feel awkward about looking different from their peers. Early menstruation can be unsettling for girls who are age 9 or younger - or who are developmentally delayed."