More accurate to say aged up because current Nahida, despite her childlike characteristics, is already an adult. If theonetruekaiser is reading this, no, this is not an excuse to lewd her. Lolicon is almost as despicable as pedophilia.

Some people also have this weird notion of "archon years", which is really not a thing. Nahida is unique in that her physical appearance slowly scales with age, but this does not apply to the other archons. Zhongli is a taken form, previously he was a dragon, a woman, and god knows what else before. Venti is a wind spirit who took the form of his friend. Raiden is a complete unknown other than that she hasn't changed her appearance at any point as far as we know.