The "teenager" is a modern invention, created by advertisers and marketing types to form their ideal customers: cash-heavy and not too sharp.
Somehow the fabled raging hormones of today's teenagers were just considered youthful passion to be dealt with--enjoyed or exploited--in earlier times. As we know many people in history were living adult lives well before they reached what we consider "majority" today.
The myth that adulthood only "really" onsets at 25, or alternatively redefining "adulthood" as "the way people behave at the age of 30" serves the statist extremely well:
1. Set the standard of maturity to something most young people don't do.
2. "See, these people are not acting the way we want them to. Clearly they're immature, so they must be controlled."
Example: "Lots of college students drink lots and party hard. Clearly they're not mature enough to carry firearms on campus."
If you read the biographies of the Founding Fathers, when they were 18 they acted in many ways how modern adolescents do at age 18 - and yet they were treated as adults.