Tons of ways to keep your sims alive forever.
1. You can turn off aging completely in the settings.
2. You can purchase "potions of youth." in the rewards section for 1500 satisfaction points. Or get it from the cow plant by sacrificing other sims,.. you can also clone it in the cloning machine if you're a scientist.
3. You can create a vampire sim. They don't age.
4. You can plead for a sims life with death, offer death a death flower or use the voodoo doll on death to keep him from taking a sim.
5. You can resurrect dead sims by writing the book Book of Life and having a friendly ghost read it or by cooking Ambrosia and having a friendly ghost eat it.