Ra345,
I use Nraas mod StoryProgression, it's the simplest way, I think.
What I do is I put "True" on the setting "Stasis" on the households that I play, that way story progression and aging is turned off. Are you playing without mods?
I play each family for 3 sim days, and while a family is your active household they age as usual.
The only drawback I've seen so far is that the rest of the neighborhood (the households that don't have story progression off) actually age much faster than my playable households, but it's something I thought I could live with.
I hope this helps.