I wouldn't say more mature, maybe maternal or matriarchal. Storm feels like a young mother or older sister. Even to those older than her.
Yup, pretty much my thoughts. She doesn't need to be an older person, just someone with a more sage-like aura about her.
But one that still knows how to have fun. I see her as the type that can keep up with Gambit, but, like you said has that sage-like aura to her. The type of person that gives advice or people know they can come to.