Hidden Traits

If you didn't know yet already, instead of having to brew the potion of immortality, you can simply apply any sim to be immortal by applying the Immortal trait on them through cheats. Simply type in Cauldron_Potion_Immortality after typing traits.equip_trait.

However, it only makes them biologically immortal, just like turning aging off in settings. They can still die from most other deaths.

Edit: The Immortal trait actually is supposed to keep them from dying from Old Age only, but they're supposed to still age.