@Manakete
@Exeryon
You're not necessarily wrong, but remember that the average life expectancy data we have for those time periods is
heavily weighted by the high infant/childhood mortality rates (due to the same lack of medical care you're talking about). It wasn't necessarily unusual for someone to live to a hundred, even in ancient times, but getting past ~12 was pretty fucking hard, even up to the Industrial Revolution. Diseases like measles, mumps, croup, general fevers, the common cold, strep, etc. that we laugh off today with our fever reducers, antibiotics, and vaccinations could easily kill a kid back then.
If you made it to adulthood, you were probably going to be fine until at least 80, barring death by violence, accident, or your body breaking down from years of manual labor.
@Ibri
EDIT: Oh, damn, you already made my point.
@fifthtouch
It's partially a Japanese thing, which is why you see it in a lot of manga. The term 'Christmas cake' comes from a saying about how single women and christmas cakes are similar - nobody wants to 'eat' them after the 25th. The gender flipped version is "if you stay a virgin until 30, you become a wizard". More generally, there's sort of a cultural thing about how you really should have established yourself in life by 25 or so, by getting married and having a family and/or having a stable career (depending on your gender).