@RandomBuzziness I haven't messed around much with the cow plant, I always forget to feed them and they die lol, but what you get from milking one is the same as the Potion of Youth right? If so, I think that seems like a really nice theory too. I mean with her having that secret agent job, I think there are all sorts of possibilities. You could even say that her own organization was experimenting with cow plants, possibly something they found while infiltrating a lab, and that as part of her research she was asked to sample the potion or maybe she did it willingly. I think you could also say that she was part of a team researching the Potion of Youth, and that she was exposed to that too.
Plus since aging itself can be disabled within the game, maybe she was exposed to something that simply disabled aging but only for her. We can call that one the "auto age (played sims)" theory lol. I say that specific option because if she was the only played sim at one point, then turning that option off while leaving the other aging option on should stop only her aging. If we want a story to go along with it, maybe they wanted to keep her around longer for her career so they experimented with keeping her young.
I like the ideas that can incorporate actual in game elements, like the Potion of Youth or cow plant potion or even the aging options. I like to hear all theories, but when you can play the game and actually see things that would make it plausible then I just like those more.