Custom lifespans across different save files?

I believe that (without mods helping out) what is supposed to happen is that the sims' total ages will change so that they are once again in the correct range for their age stage. It's not really good for the game for the lifespans to be changed on them suddenly like that. Sims and entire worlds can become age-stuck, pregnancies can break, other metrics can get very confused.

Even though we cannot see them when no game save is running, age spans are actually stored with the other Game Options in a file called Options.ini in the TS3 user game folder in Documents. The spans are not in the game saves files, and whatever Options.ini says is globally applied to any save that gets loaded up. An arguably better approach, if it can be arranged, for ongoing games intended to have different age spans is to keep different Options.ini files off to the side and swap them in/out to the one with the desired spans in them before loading up the game. Or, alternatively, use and swap out different TS3 user game folders.