During gameplay, go into your settings, and scroll down, towards the bottom it will say life length and you can use your scroller to decide the length.

If you put it at the start so the length is not very long at all, buy a birthday cake every day for the same sim, they will get older until their an elder, then once they die wait a whiile and you will have ghosts!

Depending on their traits, they might haunt you, or might not.