I had a teen sim that disappeared somehow from his home (I may have accidentally made him unselectabe with testingcheats) and then returned right after he became a young adult. He had 21 days to go before he aged to an adult, but at that point he had another birthday and became an adult! So how do I age him back down to young adult?

You can't - so far we have no way of reverse ageing sims.

Aww, well thanks anyway. Is there a way that I can turn off aging for all sims but my active household?

Yeah, disable Story Progression in Game Options.

Ageing is borked. I had a sim who got married to someone 6 days or so younger than him, then she aged up straight to elder, no mature adult stage. I'm not mistaken, I had a prior problem with ageing so had checked this well. Once he was 6 days away from ageing up to elder, she was getting so old, so I had her take ambrosia. This put her to 0 days as an elder. 6 days later he aged up and was now two days older than her. She should have been 6 days older than him.

I also had a group of six people who I created together in CAS, put them in a house, then didn't play them. They were to be friends for my playables. One of them aged up to elder days before the rest, no idea why and it shouldn't have happened.

If you go through all the ambrosia thing, you can set him back to 0 days of the lifestage he is in, but you can't make him a younger age group.

Ambrosia, as far as I've seen does nothing to the sims age. It gives them the sparkly ate fabulous food moodlet for 7 days. . I too had thought it would age them down. I didn't notice any difference in ages, and my sims have eaten a lot of Ambrosia.

Edit: I still can't get this to work!

Well, newly created sims start out a random number of days into their chosen age group, so that answers one question.

That's interesting. I've only tried ambrosia once, but it worked perfectly and moved my almost-elder sim back to the beginning of the adult life stage.

Have you tried just eating the life fruit directly? My understanding is that this should add a day of life for each fruit eaten.

Hrm..doesn't work for me. I tried it again last night. Maybe I need to eat the life fruit directly..like you said?