What age setting are you playing on ?
You can try what happens if you age them up with the cake. They might just die of old age with that like they did in Sims 3
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Do you play on normal, short or long ?
Might just be longer then.
Reason I've asked is cause I tried if auto-age still works properly on my own game yesterday. I play on long too and one of my Sims was supposed to age up to elder. Bar was all the way to the end and yet it took almost a week for them to age on their own.
I don't know how it works with dying in Sims 4 but in Sims 3, it was 'rolled'. So each day a randomizer decided if your Sim was gonna die or not. If you were lucky, he lived on. The chances to survive decrease every day.
So if you don't want to kill them off, you might just need to wait longer. You could also just move them out.
