There's another potion you can craft, called "Potion of Rejuvantion" which allows your sim to stay their current age forever. If your sim drinks both potions, then they should stay young forever and not die from old age.
Never mind, that potion just resets their age. It's description is worded in a way that makes it sound like they'll stay that way.
So now neither of these potions make any sense to me.
@TariaC Potion of Rejuvenation likely doesn't keep their current age forever, it's only an Adept potion and would negate the usefulness of the Master level Potion of Immortality.
I'm willing to bet it does the same thing as the Potion of Youth and Age-Away Serum from GTW, it probably only resets them back to day 1 of their current age group. They'll need to keep taking it before aging up to stay in that age group.
Unfortunately i've never seen anything that rolls you back to a previous age-group or stay in an existing age group... without mods that is... or being a vampire.
I tested it in my own game and thought it had worked. Also by the way it's description was written "A potion to allow a Sim to remain at their current age." I thought it meant they would stay that way. But you are right it just resets it.
I'm sorry for the misconception.
