A teenager, or teen, is a young person whose age falls within the range from thirteen through nineteen (13–19). They are called teenagers because their age number ends in "teen". Someone aged 18 or 19 is also considered a young adult.
Usage by ordinary people varies, and also varies in different societies. Most societies traditionally had a formal ceremony to mark the change from childhood to adulthood.
In the United States, younger teenagers go to middle school while teenagers from the ages of 15 through 17 typically go to high school. In the United Kingdom (UK), teenagers and non-teens are mixed in secondary school. Teenagers attending secondary school (high school in the US) generally graduate at the age of 17 or 18.
Timing of puberty [change]
On average, girls begin puberty at ages 10–11; boys at ages 11–12. Girls usually complete puberty by ages 15–17, while boys usually complete puberty by ages 16–17. The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12–13; for males, it is the first ejaculation, which occurs on average at age 13.
In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys. This can be due to improved nutrition resulting in rapid body growth, increased weight and fat deposition, or eating meat from animals which have been dosed up with oestrogen.
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