If everyone worked from 12 till 100 and the midle half of the population makes 24 hours the under and over aged would have to work 12 or so. 2x6 hours, 3x4 or 4x3 maybe 6x2.

You learn much more if you start young and keeping the expertice on the floor longer would also help.

If we reserve the younglings earnings for specific purchases productivity would increase even more.