IMC
a) I don't use the full PC class write-ups for NPCs
b) Few NPCs have any magic, so eg the ability to cast Revivify is rare
c) NPCs don't progress on the PC XP table
d) NPCs don't get balanced encounters (heck, PCs often don't either!).
If the reverse of all those is true then a world full of 20th level NPCs is plausible, it'll resemble a MMORPG where everyone else who's been around a while is max level and playing the end-game content.
a) I don't use the full PC class write-ups for NPCs
b) Few NPCs have any magic, so eg the ability to cast Revivify is rare
c) NPCs don't progress on the PC XP table
d) NPCs don't get balanced encounters (heck, PCs often don't either!).
If the reverse of all those is true then a world full of 20th level NPCs is plausible, it'll resemble a MMORPG where everyone else who's been around a while is max level and playing the end-game content.