Researchers from Cedars Sinai Medical Center in Los Angeles compared 6,200 healthy boys with a similar 1970s study.
In the study, reported in the Archives of Pediatrics and Adolescent Medicine, they measured height, weight, testicular volume, penis length and circumference.
Study leader Dr Fnu Deepinder: 'Studies done several decades ago in the same population reported that a leap forward in sexual development occurs at ages 13 through 16.'
'However, our study indicated that this spurt takes place between 12 and 15 years old.'
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.