We have a little more clarity about story progression in The Sims 4.
Townies and other Sims age and die while time passes. Though they will not make any life decisions. So they won’t get new jobs, marry or divorce and they won’t have children. If you want that to happen, you would have to go and control those Sims. When a family has died and there are no Sims left in a home, a new family will move in. They will use families from the Townies list first. @SimGuruGraham couldn’t tell us yet if the game would generate families with children.
You have a few options for aging. You can turn it on or off, but you can also set it so only your own household ages. NPC’s will age even when they are in a part of the world that you have not loaded. They will keep on simulating in the background. Unless of course you have aging turned off.
Sims won’t randomly move away. Though if you wanted them to move out, you can always do that yourself.