First, before entering the lot (meaning, you are in the neighbourhood), type
. This is necessary, and you won't be able to do the cheat without doing this first.
Then, when you enter the lot, press shift plus another Sim. It doesn't matter which Sim. Look for and click it. It will appear near the Sim you clicked on, so make sure the Sim should be in a desirable place.
Click the tombstone and enable the set age cheat. Now, when you click a Sim (DON'T use shift this time), there will be a choice called 'Set Age'. From there, you can change the age to anything, an elder, baby, kid, even death!
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