Naw, going by official published 5e FR material, the only individuals with character class levels are the PCs themselves.
Well, in that case official published 5e FR material can go jump in a lake. I'll explain why in a moment...
I feel you're applying 3e & earlier norms to a game which doesn't work that way. PC-class NPCs aren't even a good way to challenge high level PCs - they don't work that well even in 3e/PF.
Why not? As long as you take the stance that there's always a (school of) bigger fish, there's no reason at all* not to use a party of PC-class NPCs as opponents.
* - well, except one: those battles are a blaming fitch to run.
And 5e by default doesn't use them.
I hear that lake calling again...
Now, to explain my response to the very first line here:
You've got a party of PCs. In most normal campaigns one or more of those PCs is going to at some point die and not come back, requiring a replacement. So, another PC is brought in. All's good so far, right?
But consider this: that PC that was just brought in as a replacement has, in the game world, always been out there somewhere waiting for her turn on stage; she didn't just grow a class and levels overnight. Which immediately means that by default the active PCs in the party are not the only classed and levelled people in the game world, if only to account for any replacements - or party NPCs, or henches - the party might require later. Not an issue, of course, if one doesn't care about internal consistency in the game world...but some of us do.
Further, consider this: if you're using training rules, who or what is providing the training if not other classed and levelled people who are better at what they do than the PCs are?
I've always seen it that in a typical D&D game world there's going to be lots and lots of wanna-be adventurers, some of whom (including PCs) make it and a lot of whom (including PCs) die trying.
Some people have used sports and athletes as an analogy, so let me try the same: in North America there's millions and millions of kids playing minor league sports - little-league baseball, midget hockey, weekend soccer - those, along with all the grown-up beer-leaguers everywhere, are your very-low-level adventurers. Some of those kids make it to intermediate level in their sport - Junior A hockey, high school or even small-college ball, etc. - those are the mid-level types. Some of those make it close to the big leagues - triple-A baseball, AHL hockey, second-tier soccer, etc. - those are the high-level types. And a few make the bigs, and those are the rockstars - the very high-level types around whom stories are written and franchises built.
Lan-"and there's all kinds of perfectly good in-game ways to explain non-adventurers earning levels also"-efan