I recently found out that those in South-East Asia refer to themselves as old when they're above 23 years old （￣へ￣）
Seeing that in some Asians countries they call single women that are more than 25 years old left over women I'm not surprised!
Also the kind of world they live in is archaic the lifespans are probably shorter without modern medicine
I think it has more to do with the "Adventurer" life...
since an adult is at the age of 15(ish) in most of these stories to still be an adventurer at his age is an "old man"
It's based on the old edition rules, you're middle aged at 35 which is when you start losing str and dex in exchange for wis.
Since when 36 years old is being an old man? For me an old man is someone in his 50s or 60s!