A new study has revealed that boys are reaching puberty sooner, almost a year in advance.
The research in Bulgaria showed that boys are on a faster track to puberty reaching the milestone an average year earlier than their fathers' generation.
Researchers found that boys today start to develop at 12 years old, while those in the 1970s began changing when they were aged 13.
The findings suggest that trends towards earlier puberty aren't limited to girls, who have already been shown to be developing sexually at increasingly younger ages.
Researchers from Cedars Sinai Medical Center in Los Angeles compared 6,200 healthy boys with a similar 1970s study.
In the study they 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," the Daily Mail quoted study leader Dr Fnu Deepinder, as saying.
"However, our study indicated that this spurt takes place between 12 and 15 years old," Deepinder added.
The team found that boys' testicles did not grow substantially until the beginning of puberty, around age 11. However, penises appeared to grow gradually from birth to sexual maturity, starting at around 2 inches and reaching an average length of nearly 4 inches 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 old.
The study fund that boys of the same age in the 1970s study had relatively smaller genitalia. However, these size differences disappeared between the two generations by the age of 17.
Dr 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.
The study has been published in the Archives of Pediatrics and Adolescent Medicine.