So I have Ultimate Collection and use Inteenimator.

Aging was working just fine until recently, when my custom sim Heri could age normally, but Ripp Grunt, Johnny Smith and Ophelia Nigmos were not able to age normally. Even Buck Grunt won't age at all.
If it helps, I moved the teens and Buck in with Johnny Smith using the Sim Manipulator's Family Manipulator.

Are the sims reaching their final day(s) of that age without popups (unless modded out), or isn't the cake accessible, or are you waiting for them to grow up on their own without using cake?
Or something else?

Is it just the sims moved in via mod, or is aging affected for other sims too (other households)?

Are other things affected for those sims (pregnancy progression, toddler/infant care, only access to old wardrobe...)? Any of those would suggest they haven't been added properly to the family.

Not sure with the SimManipulator, but for the Insimenator you have to spesifically "add to family" for sims to be properly added (spawning in sims and making them selectable isn't enough). If they aren't added properly, some things aren't available by default (default aging, care for toddlers/infants, pregnancy progression, new family's wardrobe, etc.). When they're added, they disappear from that menu (family tree object).
Maybe check again if you used the proper option for the SimManipulator?

Could also be the Inteenimator - teens/independent teens and growing up options for those.

This is a stupid obvious thing that you've probably checked, but...those are always the things that trip me up, so. Are you absolutely certain aging is on on that lot?

Aging is on in the lot, I found the answer myself. Apparently, I have to spawn their biological clocks to get them to age normally when they won't age at all. Spawning their biological clock fixed the aging issues.

I'd recommend reading the documentation that came with inteenimator, It explains how to use it and how to solve issues.

Helpful advice, but the User Guide and Mod Conflict Strategy guide it came with doesn't mention a bug where certain sims do not age. As I said, my custom teen on the same lot was aging just fine.
This is one of those issues where Reddit seems to help and explain what is going on, not the official User Guide.

Pretty sure there's a bit in the user guide that says to spawn biological clock for sims with aging problems. Maybe I'm mistaken, glad you got it solved anyway!

Edit: Oh yeah- it reads as though it's talking about the townies, but I think it probably means any town with such sims.

Huh. I wonder if that's how I once ended up with Vidcund Curious showing as a teen in the UI, while looking like an adult (had to grow him up for him to become a proper adult). Only happened once, and it's been years, but I found it so weird I still remember it. Can't remember if I was using Inteen at the time, but it's very likely (I've since removed Inteen).

I remember that if anyone was having a problem with specific sims with Inteen, the solution always seemed to be to spawn the biological clock!

I must have missed it, I do remember seeing something like that though. Thank you.
That also explains why *certain* sims weren't aging - namely, the default teens that came with Strangetown - while my custom sim was able to age normally.

It's a complex mod, it was a lot easier to use when the forum was still up and we could ask the creator questions. I'm glad the person who helped you on reddit knew what to do!