Most, if not all of my OCs are way over 18 (usually mid twenties) since I have a really hard time writing teenagers and don't feel too comfortable with it.

The youngest 3 I have are all in sci-fi settings since they're all aliens uh.

One of my mains, Scias Zeizan is around 19-20. He's part of a species of changeling type galactic peacekeepers that get sent to other planets to observe alien species like a bunch of space cops. He's a huge moody grump most of the time and has a hard time dealing with most people, especially humans, at first. He's kind of edgy but I like it because he's great for comedic situations.
I don't change his backstory much because it basically lets me stick him in any modern or sci-fi setting.

A'meto Arkenia's probably the only one of two I have at 17 or 18 depending on setting, but in his origin universe, he's an alien prince and scholar. He comes from a warrior species of moth-insect aliens and he gets a lot of grief for being a complete doormat. I'll probably change him around heavily depending on setting and plot.

The other is Katia Korsak who is the daughter of another two of my mains, one who is an android space captain and the other is a reluctant warlord king of a country of warrior moths. Katia's half human and half alien-moth and usually is an engineer. She really likes messing with robotics and likes to spar people.

I have a human version of her from an AU universe by the name of Katya Ann who's 18 but I've really not gotten to use her much and they are basically the same personality and job, just with slightly different backstories.

katia/katya sounds like Nicote and Alexander Oliver, same guy, alexander oliver's just Nicote in the horse world.....or his alter ego XP

I could probably work with that, I got a lot of STEM kids I can age a little
I usualy don't do above 20 tho cuz....well, collage :P

Katia's more wholesome that I usually write in characters, lmao, so that works for me, just need a plot to work with.

hmmmm
technically many of mine coem from an elemental planet called Terrarine

Sounds interesting? What's it about?

Canonically Katia is from an organization by the name of the United Galactic Federation which functions similarly to Starfleet if you've ever watched Star Trek.
Basically like the United Nations of space and these guys go out to explore other planets and try to keep the peace.

oh that'll fit well

So terrarine and Earth are intercnonnectedish but Terrarine has a "darkness" that took over most of its world and corrupts everything (wynncraft, if you've ever played that Minecraft pc server). And everyone has certain elemental abilities and stuff.

Haven't played wyncraft but that sounds interesting.
The Discord universe Katia's from has characters with magic powers known as Aether out of meditative practice taught by an alien society that basically lets people manipulate elements in the universe they are aligned with with their minds. Katia barely has them herself (magnetism based metalbending) but she hasn't put much time into learning it since she's way more interested in building machines to fight with instead, but she's definitely compatible with magic sci-fi stuff. :P

yep that's sort alike my elemental thing too
You're either born with an element or youre born with a Gift that aligns with an element. Some Gifts are deadly, others are amazing.

So assuming Katia'll probably be travelling and could end up on their planet, or your characters end up coming to her organization for help.
Then what? What exactly is this darkness they are trying to fight?

Well actually..

here just go here and ill let you into the doc
its just easier to explain

https://docs.google.com/document/d/12UxikZXIOJfkGqchkxxbl_AQtt-41iiNvgp588cPIMY/edit?usp=sharing