The new update on the wiki Aging/Ageing site explains most of this (Can Npcs and MC die of old age? If yes is it then a % chance or at a certain age Npcs and MC dies?) guess this means that none recruitable NPCs don't die where MC and recruitable NPCs is Immortal as long that they don't retching 0 in STR/DEX/STAM and can die young if the do.