I play rotationally between many households in different worlds within the same save based mainly on my interest. Over time, I have some new generations of characters that interact with other sims in the world. It is one of the reasons that I am really flexible about pre-mades. They might get involved with my sims but I kind of have my own thing going. However, I don't judge anyone else wanting to play with any others so I'm open minded about all of the new pre-mades. Some of mine might connect with the new ones but I basically do my own thing.
In TS4, I like being able to customize aging in the settings and to manage my own story progression. Generally applying story progression to all but one household would not work for me. I do have my own concept for the passage of time which is that the calendar shifts continually but I am seeing snapshots of each household. Time is passing but it is a bit gradually. At some point I'll be likely to switch on aging for a bunch of unplayables. Not all of my sims will reproduce especially any more awkward born-in-game sims.
As for what I do on different lots, I actually tend to send a household together to a given lot to do activities together. If they go bowling, they all go bowling or they all go to the gym, park or pool. Sometimes I might leave one at home which isn't a big deal for me. I am more likely to turn off aging in that case so that the sim I am not playing is not "penalized" but it just depends. I don't actually think that much about it.
I tend to play a whole group of sims. If a sim lives alone then obviously I just focus on that sim. Most of them end up in families. I usually like them to do whatever the activity is together or to be on the same lot. Most of my lots do have different things to do and I edit community lots to have more features or meet more needs. If I have toddlers then I need to take them into account but sometimes the parents just send them to daycare and have a day out.
