Kids in preindustrial times started working at about fourteen 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, which is close the average life expectancy in medieval times.
Teenagers now are useless, except as cheap labor in industries like fast food, which evolved to exploit precisely this fact.
