Puberty starts somewhere in the age range of 8 to 13 years for girls and 9 to 14 years for boys. However, in Norway, in 1850, girls hit puberty at around seventeen. Over the next 100 years that age decreased to thirteen and a half, where it has stabilized, but a similar trend has been seen in pretty much all the industrialized countries, including the U.S.. No one knows quite why, but there are a number of theories, including:
- Better nutrition,
- increased stress, and
- artificial chemicals in the environment or in the diet.
It seems clear that this trend has something to do with improving living conditions. Rapidly developing countries like China are experiencing the same trend in earlier puberty right now. Wealthier areas in developing countries have girls starting puberty at the same age as girls in “privileged” countries, while their compatriots in the poorer areas do not. Also, Overweight kids tend to start puberty earlier.
However, explaining the earlier puberty is difficult because no one knows for sure what exactly triggers puberty to begin with. The genetic switch that tells the hypothalamus to start the process probably involves multiple genes that are affected in complicated ways by how and where a person grows up (when the environment affects how genes are expressed it’s called epigenetics; NOVA has a nice little program that explains how epigenetics results in differences in identical twins.).
Increased stress might be another explanation. Girls in Bosnia and Croatia started having puberty later and later during the war in the 1990’s. However, it appears that other types of stress, such as from insecure relationships with parents and adoption, can do the opposite and trigger even earlier puberty (note: really early puberty in kids as young as 9 is called precocious puberty and is a growing problem).
Certain artificial chemicals that disrupt the endocrine system, which is responsible for hormone production, have also come under suspicion, but their effects have been hard to prove.
Whatever the reason, the earlier onset of puberty has lead to an increase in the length of adolescence (which tends to start with puberty and ends somewhere in the mid-twenties). It’s hard to say though, if all the extra time is beneficial, since it does give the developing brain extra time to adjust to a more complex society, or if it just makes for a longer period of trying times.