Nero came from a different era so it is understandable that you would say she's hasn't age, but in the anime and the manga already stated she was from like centuries ago, "she hasn't age" yes of course she hasn't because she used forbidden magic i think like 500 bc ago? i don't remember much, i mean it's kinda debatable if you think about it i don't think they really stated if she really is young at least 15 years or 16 or just really old but still looks young, but i'll stay with my state if i see evidence....anyways.