As a general rule, for human NPCs in games I tend to assume a default of being 0-level, with a single 1d6 HD. This includes anybody of any profession or background who isn't likely to have 'levels' in the strict sense - whether a serf or a king - because he's probably never gained the necessary experience. Magic-users, clerics and high-level warriors will have actual levels, just like PCs. Non-human NPCs simply have monster HD.
Why? I'm really not sure. It was simply ever thus, and I've never rationalised it. Nor have I ever really rationalised levels at all. Why not have everybody, PC, NPC, and monster, use HD, varying the dice used for PCs according to their class? Is it just because it feels better to say "I'm 3rd level" than "I've got 3 HD"?
Or is it because PCs are special in some sense - different from the rest? In a G+ post I mused that perhaps there are two categories of person in the world. There are the vast mass of humanity, who do not have levels and indeed cannot - although they might improve their skills slightly and/or come to possess fortunes and great power - or be born as royalty for that matter. And then there are the special ones, maybe minor demigods, plane-touched, faerie-blessed, dipped-in-the-River-Styx, annointed by storm giants, etc., who have this innate capacity to be something more than human (which a high level D&D PC undoubtedly is) even if they don't realise it.
I don't like the idea particularly, because a) the idea that D&D PCs are nobodies like anybody else but make it by applying themselves is innately appealing; and b) it sounds a bit like the plot of a bad YA novel. As a matter of fact I think it's a cabbage of an idea, and no, that's not even a remotely pathetic attempt at tying together an Alice Through the Looking Glass gag.