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Published 16:18 22 Jul 2018 BST
Updated 17:45 22 Jul 2018 BST

"That would include everything from exercise to junk food to TV to chemicals."
Dr Frank Biro of Cincinnati Children’s Hospital added: "If kids are looking older, it means that parents should be monitoring them, because that superego doesn’t kick in until late teens or early 20s.
"The kids need a hand to know what they’re doing," he said.
The study also revealed that girls are hitting puberty much younger, with some developing breasts as young as age eight.Explore more on these topics: