There's childhood, adulthood, and the messy bit in between. Here's how we've defined adolescence throughout history - and why it's time for a new category.

The psychologist Jean Twenge of San Diego State University notes that teens are growing up more slowly by many measures, compared with their 20th-Century counterparts. A typical 17-18-year-old in the US, for example, is now less likely have tried alcohol, have had sex, or acquired their driver's licence, compared with similarly-aged teens only 20 years ago. A 13-14-year-old is less likely to have a job or to have gone on dates. Meanwhile other measures of early adulthood, such as teenage pregnancy, have reached historic lows in the US and Europe. Twenge points to a number of reasons why growing up is slowing down. There's little doubt that technology and the internet has played a major role, meaning more interaction with peers happens online and in the home, where sex, experimentation and trouble are perhaps less likely. For this reason, she calls this latest crop of young people the "iGen" generation, and has written a book all about their characteristics. But she also points out that some of these trends were already beginning before the online culture of the 21st Century, and so the internet can't be totally blamed. Her hypothesis is that teens behave differently depending on how hostile and unforgiving their local environment feels to them, an idea that social scientists called "life history theory". In tougher times in history, teens were forced to take a "fast life strategy", growing up faster, reproducing earlier and focusing on basic needs. Now life in the West is generally more forgiving, and families are wealthier – at least on average – so it's possible for teens to take a "slow life strategy", delaying the transition to more adult behaviours.

By many measures, adolescence now continues until around the age of 24 to 25

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