@lucyhannah85 Some of the NPC's have been granted a sort of 'eternal life'. This means that you won't be able to cull them, It also depends on your own gameplay settings. When you only let the played household age than other Sims will not be able to age up. There is a mod with which you can alter the state of any of your Sims and also NPC's. Google for MCCC.
You could also check in the manage worlds section, see of any of the Sims you don't want in your game resides 'not in the world' and delete these households or manage them the way you see fit.
Have fun playing 😉