I still don't understand aging

@Kanelbullet Can I ask what all of your aging settings are? 

• Your game's aging is off for active sims ("Auto Age (Played Sim)" to no), correct?
• Is aging off on non-active sims/households - non-active sims ("Auto Age (UnPlayed Sims)" to no/unchecked box)?

I'm not sure if this is what you are looking for but I play with aging off for active sims ("Auto Age (Played Sim)" to no), as I then have the most control as to when I age them up. I also play with aging off on non-active sims ("Auto Age (UnPlayed Sims)" to no/unchecked box). This way no one ages up unless I go into their household and have them age up. I use the cake to age them up.

Aging is on but only for active sims.

BUT! However! I waited a sim night while playing the household that got the call about a birthday party. Then I started the other household figuring the sim had aged, because he did call and say it was his birthday. But! It was not until the next day! So I guess ... if you say no to going to the party, then the sim won't age?

@Kanelbullet As far as I know if you have aging only on for the active household the only way for other households to age is if you go in and play them. This article may be helpful in explaining some of the details more thoroughly: https://www.carls-sims-4-guide.com/tutorials/sims.php

Ok, there are TWO settings. "Auto Age (Played Sims)" and "Auto Age (Unplayed Sims)"

When you look at one of your towns, populated houses will have a circle with a plumbob above it. If the Plumbob is bright green, then that is considered a "Played" household.  "Played" doesn't refer to a household you are "currently" playing. It refers to ANY household that has that bright-green plumbob. Houses get that the instant you go into that lot in "play" mode. Even for a second.

"Unplayed" households are all the others with dim Plumbobs. You can remove them from being "Played"  by using the household manager. Just view the household and click on the green plumbob. It'll turn dim and it'll be no longer "played". You can still go back in and play them, which will turn the plumbob bright green again.

By viewing households in the manager, you can see which ones are "played" and not.

Now, those two settings. Auto Age (Played Sims) has three settings:

Yes - This will turn on auto-aging for ALL "played" households. (even if you aren't actively playing them)

No - This will turn off auto-aging for ALL "played" households. (even if you aren't actively playing them)

Active Only - This will only turn on auto-aging for those "played" households that you are currently playing in "live" mode. If you aren't actively playing the household, they won't age.

Auto Age (Unplayed Sims) only has a checkbox. They refer to any household with the dim plumbob. You uncheck it and any household with the dim plumbob will not auto-age.

That's how the vanilla game works. However, if you have a mod like MCCC, ALL that changes. MCCC can change how all of that works and makes the entire system a LOT more complex.