In a low-magic world, I'd imagine that most threats would be from classed character races or other low-CR humanoids with levels.
Personally, I don't like the way that "D&D Demographics" interoperates with the normal D&D magic level... but I changed the other side of it, the demographics.
IMC, natural life expectancy is similar to modern life expectancy. However, there are a LOT of monsters which like eating people, and enough people die in "unrecoverable" ways to keep the good human(oid) population low.
With a small starting population, it's easier to threaten whole communities -- and that's a real threat, because "it takes a village to Raise a child".
In terms of level distribution, a dangerous D&D world has few (surviving) Commoners. They just aren't viable. My world has Apprentices being people of level 1-3, Journeymen being of level 4-9, Masters being of level 10-14, High Masters of level 15-17, and Legendary people of level 18-20. Every community has a Master or two, and every city has many Masters and a few High Masters. Important cities have Legendary people. Most citizens are Journeymen.
The two NPC classes I kept are the Expert and the Aristocrat. Most "worker" NPCs have Expert levels, but also use PC class levels to boost their effectiveness. In a farming community, most farmers have at least one level of Druid, and there are always a couple of Druids high enough level to cast Stone Shape and Plant Growth. Communities without such a resource just aren't viable.
Everyone is in the militia. Everyone has had some combat training. Communities survive because they are NOT threatened by hordes of 1st level goblin Rogues.
-- Nifft