Anyway, I'm pretty sure people are allowed to stay children for longer nowadays, instead of being told to marry and work in the coal mines in their early-to-mid teens.
More cynically / conspiratorial / on the other hand, people are kept infantilized as well as a means of control by various parties in whose interest it is to keep a population docile and dumb. Can't overthrow the government if you've got scantily clad teens to watch.
For example in Britain (age of majority 18, 16 in Scotland)
Sex: 16
Marriage without parental consent: 18, except Scotland (16 for local elections, 16 for Wales coming in next year)
School leaving: 16, but must remain in education outside of school (say official apprentice) until 18
Drinking: 18, 16 in public with a meal when bought by parent, either 0 or 5 at home
Driving: 16 moped + tractor, 17 cars, some vehicles are even higher (23 or 24 for powerful motorbikes for example, 21 for minibusses unless in the army)
Smoking: 18 to buy, 16 to smoke in E+W, 18 to smoke in Scotland
Gambling: 18 (lottery was 16 until recently)
Voting: 18, except in some caes in Scotland where it's 16