Just thought I would let you know; I have a self-made mod that does the same thing, and I've found that there isn't a problem aging up if you have NRaas installed. I don't know if it's MC or Error Trap, but the mod will automatically reset the Sim on aging up and the aging up will occur correctly. The Sim will keep his/her current traits, and new traits will be added as well.
That said, any townie aging up WILL reset when he or she ages. (I think your own Sims do as well.) Since you can't age up while you have actions queued up (at least I don't think I've ever been able to), being reset isn't a huge deal, but it does happen.
If you don't use NRaas I can't say the results, since I've never played a game without Twallan's mods.
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