Boys today may be on a faster track to puberty than their fathers' generation.
The findings suggest that trends toward earlier puberty
aren't limited to girls, who have already been shown to be developing sexually at increasingly younger ages.
In the study, researchers from Cedars Sinai Medical Center in Los Angeles compared 6200 healthy Bulgarian boys with a similar 1970s study. Both measured height, weight, testicular volume, penis length and circumference.
"Studies done several decades ago in the same population reported that a leap forward in sexual development occurs at ages 13 through 16," said Fnu Deepinder, who led the study.
"However, our study indicated that this spurt takes place between 12 and 15 years old."
Deepinder and his colleagues found that boys' testicles did not grow substantially until the beginning of puberty, around age 11. But penises appeared to grow gradually from birth to sexual maturity, starting at around 5 cm and reaching an average length of nearly 9.5 centimeters by the age of 19.
However, both penises and testicles grew most rapidly between ages 12 to 16, while boys added the most inches in height and gained the most weight between 12 and 14 years gold.
The study also found that while boys of the same age in the 1970s study had relatively smaller genitalia, men are not necessarily more well-endowed today, with size differences disappearing by age 17.
Boys in rural areas had slightly longer penises than their urban peers, although the difference was only about 0.4 cm.
Deepinder said that genetic, environmental, nutritional and educational factors could all be behind the faster development today, but it remains unclear what impact earlier puberty might have on men's health.
It also remains unclear whether specific differences exist between populations from different parts of the world.
Why do you think boys are reaching puberty earlier than before?