Boys are hitting puberty an average year earlier than their fathers' generation, a study says.

A study in Bulgaria found boys today start to develop when they reach 12 years of age, while those in the 1970s began maturing when they were 13.

The findings suggest that trend towards earlier puberty isn't limited to girls, who are developing sexually at increasingly younger ages, reports the journal Archives of Paediatrics and Adolescent Medicine.

Researchers from Cedars Sinai Medical Centre in Los Angeles in the US compared 6,200 healthy boys with a similar 1970s study, reports the Daily Mail.

They measured height, weight, testicular volume, penis length and circumference of the subjects.

Study leader Fnu Deepinder from Sinai Centre said: "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."

The team found that boys' testicles did not grow substantially until the beginning of puberty, around 11 years.

However, penises appeared to grow gradually from birth to sexual maturity, starting at around two inches and reaching an average length of nearly four inches by 19 years.

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 of age.

The study found 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.