I'm from Slovenia, where it's exactly the opposite. Being born in November, I was among the younger people in my class. The cut-off point was January or February (of the next calendar year). So it could be that the more stress you have, the longer you live (better personal/immune system development). Alternatively, younger classmates (i.e. people that go to school with peers that are in average older) achieve accelerated development (because they hang out with "more developed" kids) which could also somehow affect life expectancy.