I think there are several reasons for keeping teens and twenties children younger- I don't mean it's a good thing, but it's an understandable one. The first is you can't work at a younger age like when I was a teen, I got a job as a waitress at 13. No-one but no-one hires people under 16 now, even if it is technically legal, and many prefer to wait for 18 because it's easier in terms of risk assessment. Even charity shops and the Duke of Edinburgh scheme have been hit by this- charity shops now pretty much won't take under 16's at all, which again is a shame, they don't get experience of having a small job, earning their own money, interacting with the public. We just went from job to job at that age, and so were out of the house and much more independent from day one.
Second, and rightly I think, the government has brought us more into line with other countries in terms of expecting 16-18 to be in training or education, with higher education extending til after 22 (including a Masters which the gov't will lend you money for). These student need paying for, and so continue the teen relationship longer than might have been in the past- in Germany this responsibility extends til your child is 25!
Society has structured it so that a) training and education takes much longer and working is deferred and b) it's hard to get on the property ladder and c) we don't marry and have babies young like my mum at 21, so the natural points of maturity have all shifted later.