No, there's no limitation to age provided the character can take care of themselves or has someone who is willing to watch over them (if not player characters, it's likely one of the NPCs would). In Anna's case especially, she would be an acceptable character.

While there are no limitations on that sort of thing, it would take a while (per player discretion) before she was able to fully use and control her powers. If this isn't something you wish to play out (especially since Anna is prone to accidental outbursts), one of the Dragon Mages could have placed a limiter on her until you're ready to play things out. So long as you express everything in her permissions post so that other players are aware of what they're getting into, you should be good.