"Huh, a Normal lifespan goes by a lot faster than I expected," I said. "I'll just set them to Long." And all would be well, if the game hadn't added some arbitrary number of days to everybody's ages as a result. My active sims are "older" than they should be (i.e. their aging meters on the Simology panel jumped forward quite a bit) and last time I tried changing the age span when a save was already established I got spammed with Stuck Aging Check messages a couple nights later. How do I get the game to leave everybody at the relative age they're supposed to be, or if that's impossible, does anyone know whether it's a specific amount that gets added so I don't have to just guess at what to take off of everybody's ages with MC?

I hope this made sense!

I'm afraid this is almost never a recipe for success. The game isn't likely to care if the age spans ahead of a certain sim are extended. But if the ones behind them are and the sims' ages are kept intact, then suddenly you will have YAs who are only old enough to be children or teens, etc. so the game has to do something with them. Is this what you are seeing?

I don't know if this would work well either, but I would take a different, more gradual approach. I might start by extending the elder stage, that isn't going to bother anyone. Then work backwards by using MC to shift younger elders up in age so that by extending the adult stage slightly, no one would be in a badly defined place. Then shift the younger adults up and try to do the same with the YA stage, and so on moving backwards all the way to babies.

But still, I believe the general recommendation is to not do this, at least not all at once across the board.

When you change the Age Stages all sims have there ages adjusted to how far along they where. So if your sim was a YA and you originally had the Age Stage for YA's at 50 days and your sim was at oh say 25 days or half way. And if you were to change the YA Stage to 300 days then your sims will have their age adjusted to the half way mark of 150.
You can change the length of the age stages at anytime as long as you make it longer not shorter the game doesn't seem to adjust them properly if you try to shorten the life span.

Yeah, I changed the Age Stages through the options menu like normal. An annoying thing that I discovered when I went to do that, also, was that somewhere along the line it had gotten reset to the default Normal lifespan instead of the tweaked one I'd been using which was a little longer (though not as long as I decided today to set it to)- quite possibly *that* explains why things were a little funky. I definitely didn't do that intentionally, but after a recent Windows update I found all the other game settings had reset to default, so that must have happened then. Gonna backup my Options.ini to avoid a repeat.

As for this save, it's still pretty young at less than an in-game year and I'd have to go back and change a bunch of Sims' life stages to get everything back how I want, so I guess I'll roll back to the start. This is why I always save a backup copy when I've finished setting up the neighborhood, before I go into Live mode for the first time...