Just a little random thought. I know that Sims that come with DLCs will have a "X Days until Age Up", but I wonder if it's possible to just..set that up for a Sim yourself? Like a code or anything?

You mean change the number of days to a value of your own? I don't know of any mod that excalty does that, other than resetting your progress back to zero, or just pausing your aging for a while by flagging with MCCC.

Or without mods or cheats , there's numbers of way to reset, or even live longer with bonus days, from potions, wishing well, or fulfilling a certain aspiration goal, and probably a few more...

With MC Command center you can adjust the ages by lifestage to whatever you want. https://deaderpool-mccc.com/#/releases
You can also reset the age of sims. If you want a sim to age up before the end of their lifestage, all you have to do is throw them a birthday party. If the sim is outside your household, and if you do not use MC Command Center.. You can use Littlemssams Let friends age up mod. https://littlemssam.tumblr.com/post...-with-aging-off

I see, thank you. I know about the ways to reset / force an age up, but I was more thinking that II could have a Sim at a random progress or so into their age up. So sort of...space out the aging process, you could say? And maybe pretend that their birthdays are on different days? If that makes any sense at all. It was just a passing thought, anyway! My game doesn't hinge on it being a feature, just something that would be interesting if I could do it!

actually, you can set the age of npc's by clicking on them, either by using flags to "never age" or using the uhhhh tuner (?) to set age, aging them down or up.

It's in the Commands section