So I have basically a household of three sibling witches that just turned elder. The youngest one, which is an elder now, stopped aging for some reason at some point (don't remember) and I had manually transition his growth. Why is that Sim doesn't age in my game when playing the household and how do I resolve this? Somewhere under the nraas story progression menu I had set him to age true, but that had no effect what so ever. I suspect this could be glitch, but my curious mind would like to know if I somewhat interfared with something and whether this is a hidden gem feature or something.

Check his Life Time Rewards and see if he recieved the Age Freeze Potion.
He will only age after that if you give him a birthday cake or remove the reward.

I don't have Generations installed, so that's not that possible. And the thing is, I already aged him to an elder. I'm not sure how's code wise it's any different having him blow out the candles than transition to a next life stage with testingcheats. And to be clear, I want him to age, not just die from old age on my command.

Hmm, that was my best guess.

I did find an old Yahoo answers thread where someone had this issue.
They suggested sending the affected sim on vacation for a day.
I guess they started aging when they came back.

I'll have to try that out. Thanks.

ETA: Still hasn't worked.
ETA2: Seem like moving to a new town (not the reason why I did so) also seem to continue its aging, but I'm not sure if temporarely as I played for longer than a sim day.