The cause of this problem is the same as the cause of so many present ills: specialization. As jobs become more specialized, we have to train longer for them. Kids in pre-industrial times started working at about 14 at the latest; kids on farms, where most people lived, began far earlier. Now kids who go to college don’t start working full-time till 21 or 22. With some degrees, like MDs and PhDs, you may not finish your training till 30.
