NYT has a long spread to expound on many theories, all of them valid. But this paragraph alludes to a theory they missed:
In the late 1980s, Marcia Herman-Giddens, then a physician’s associate in the pediatric department of the Duke University Medical Center, started noticing that an awful lot of 8- and 9-year-olds in her clinic had sprouted pubic hair and breasts. The medical wisdom, at that time, based on a landmark 1960 study of institutionalized British children, was that puberty began, on average, for girls at age 11. But that was not what Herman-Giddens was seeing. So she started collecting data, eventually leading a study with the American Academy of Pediatrics that sampled 17,000 girls, finding that among white girls, the average age of breast budding was 9.96. Among black girls, it was 8.87.
For Hispanics, it would be somewhere in between, and for Asians, it would be very close to or slightly later than the white average.
There’s another explanation: The reason why the average age that American girls (and boys) hit puberty is decreasing is that more and more “American” girls, as a percentage of the whole, are black and Hispanic. This might also explain why the average gestation period for expectant American women is slowly decreasing.