If you have full aging on they eventually all die and your game will be left with nothing but poorly dressed random townies. I learned this the hard way on my first attempt at playing a legacy in the Sims 4.
In order to prevent this in my game I had to change how I play. I started a new save and I now have aging off for everybody except the current household and I rotate through all of the families one by one so they all age and progress at the same pace.
I know I can use MCCC story progression for this but honestly I don't like it. It just fills my game with poorly dressed sims when all the uncontrolled sims start having children and they grow up. I'm to much of a control freak and I want all the sims in my game world to look nice and not be dressed in crazy random mismatched outfits.
