It looks like a back-of-the-envelope calculation to me: twelve summers before the developmental changes she mentions divided by sixteen, when they start driving, get a summer job, start dating, etc. You could probably plot a curve of hours per summer, starting to decline at twelve and declining fast after sixteen...
exactly -- it's almost like the author just made the number up out of thin air or something...
I'm sure it's within a +/-25% error range.
I don’t.
Quite. It's too depressingly plausible a thought as it is, without digging up evidence that it might actually be true too.