They should be as strong as the story requires. There is no
concrete answers here, just world building, which is, ultimately, on you.
Let’s start with your main character. Elves (or Elfs, if you
prefer) aren’t real, so they don’t age at a fixed rate. Which means, saying he’s 16
years old isn’t that useful. I understand the intent behind your
statement, but it’s probably important to step back for a minute.
If we’re taking D&D’s setting basics, and running with
those, a 16 year old elf is a small child. Conversely, D&D’s perfectly
happy to call a 16 year old human an adult. This is, of course, assuming your
setting’s elves aren’t completely ageless, come into existence fully matured,
and then never change.
It’s worth remembering, when you’re building a fantasy
setting, that you control all of the
variables. Sure, your human characters should, probably, come across as mostly
human, in most cases, but even that’s not set in stone. Accusations that
Aragorn is unrealistic are fairly rare, and this is a character who’s in his
80s. (And, yes, there’s an entire internal justification for that, but Tolkien’s
race of Men aren’t really human. They’re another flavor of mythical beings,
like his elves and dwarves. Aragorn is a step further from that, but the point stands.)
When you’re talking about elves, that’s a very open topic.
Depending on your source of inspiration, that could be anything from beings
that are basically human characters, that have access to very advanced magic or
technology (and no, this isn’t an oblique Stargate reference), a variety of fae, normal
people who’ve been altered by some release of magical energy, or just another sentient species wandering your world. It’s up to you to define who and what they are, in your setting.
This also spills over into what sets them apart from a “normal”
character. What your elves are
is influenced by what you want to talk about. (Because your main character is an elf, their nature is far more important than if they were a minor side element in your setting.) Once you have that, then
you can start to extrapolate how your elves are different from other beings in
your setting. This could be as simple as your character being lumped in with
the other mythical beasts and viewed as a different flavor of monster by the
people he’s trying to save (or not), or it could be a coming of age story. This
will seriously influence what your elves are. How alien they are. How they age.
What their society looks like. It also affects how strong they are. Depending
on what you’re creating, it’s entirely possible your character is already a
superhumanly powerful engine of destruction by 16, whether he has the emotional
maturity to handle that or not. In turn, that would seriously influence how
elves are perceived by others in your setting. Or, he could still be a small child. Where he lands between these points is something that needs to fit the story you’re trying to tell.
To varying degrees, the same is true of the other creatures
in your world. If they’re supposed to be incredibly powerful, to the point that
normal beings can’t even slow them down, the apocalypse is an extinction event
in motion, then that’s your answer. If they’re more of an environmental hazard
that a well equipped group can deal with, again, that’s your answer. If they’re
a nuisance that only becomes a serious problem in large numbers, you get the
idea. In practice, you’ll probably want a mix of these things, depending on
what your setting needs. It’s entirely reasonable that you’ve got fairly common
threats like ghouls that can be dealt with, while still having far more powerful
beings like titans or leviathans wandering the world wrecking things. How these
interrelate will be influenced by the story you want to tell.
World building starts with the idea of wanting to tell a
story, and having a vague idea of what you want to talk about. Then
extrapolating a world that supports those ideas. Finally, you go back through
and start nailing down the fine details, like, “how powerful are these monsters?”
or, “how did people react to their arrival?”
This leaves me in a slightly awkward place: without knowing
what you’re trying to do, you’re asking for some of the final detail work
without knowing what you wanted to do in broader strokes.
At a very basic level, the more powerful the creatures are,
the more severely isolated human communities will be. I’m using power as an aggregate
here, endless swarms of easily dispatched monsters that will overwhelm and
obliterate can be more effective than a skyscraper sized
behemoth that shrugs off any injury.
At the extreme end, humans may be restricted to a handful of
small enclaves, and extinction could be imminent. On the other hand, you could
easily have a setting where survivors have retaken and fortified entire cities,
with heavily armed caravans wandering between, and smaller enclaves scattered
across the world.
It’s entirely possible you’re setting up an environment like
The Witcher. There are monsters, but
they’re more of a pest than a real threat, and the apocalypse which unleashed
them on the world is a dim memory.
There’s an old cop-out answer on physics exams, “the
problem cannot be solved with the available information.” That seems to apply
here. When you’re building your world, you have the ability to shape it to fit your narrative.
Think about the kind of story (or stories) you want to tell in it, and build
your setting accordingly.