Puberty Starts Earlier Than It Used To. No One Knows Why.

Several factors are most likely contributing at once. And many of these issues disproportionately impact lower-income families, which may partly explain the racial differences in puberty onset in the United States, the researchers said.

A new normal?

For decades, medical textbooks have defined the stages of puberty using the so-called Tanner Scale, which was based on close observations between 1949 and 1971 of about 700 girls and boys who had lived in an orphanage in England.

The scale defines normal puberty as starting at age 8 or above for girls and age 9 or above for boys. If puberty begins younger than those cutoffs, doctors are supposed to screen the child for a rare hormonal disorder called central precocious puberty, which can spur puberty as early as infancy. Children with this disorder often undergo brain scans and take prescribed puberty-blocking drugs to delay sexual development until an appropriate age.

But some experts argue that the age threshold for alarm should be lowered. Otherwise, they said, healthy children could be referred to specialists and undergo unnecessary medical procedures, which can be physically taxing and expensive.

“There’s plenty more data that age 8 is not the optimal cutoff for separating normal from abnormal,” Dr. Kaplowitz said. In 1999, he argued that the age cutoff for normal puberty should be lowered to age 7 in white girls and 6 in Black girls. “That did not go over too well,” he recalled.

That stance, though, was bolstered by a recent study from Dr. Juul’s group showing that, of 205 pubertal children younger than 8 who underwent brain scans, just 1.8 percent of girls and 12.5 percent of boys had brain abnormalities indicating central precocious puberty.

But lowering the age cutoff remains controversial, with many pediatricians arguing that the risk of a disorder is still large enough to justify extra precautions. Others, like Dr. Herman-Giddens, say that the changes are a sign of a legitimate public health problem and should not be accepted as normal.

“It might be normal in the sense of what the data are showing,” Dr. Herman-Giddens said, “but I don’t think it’s normal, for lack of a better word, for what nature intended.”