Well given its a fantasy world, and most fantasy worlds typically are an idealised version of medieval times, so it incorporates knights, lords, magicians and various creatures and non-human races. Then we have to consider the fact that in such a world, as with in the real world of the dark ages, people were usually considered of age when they hit a certain age, usually much younger then today. Boys became men at around 16, girls became women around the same time, mainly because you werent expected to live past 35 or 40 years old.So taking all that into account we then have to consider cultural milestones, something that in her culture marks her out as an adult. For Dwarves you could go with the obvious, can she swing a sword or axe with enough skill to kill so many opponents in a row, or can she forge an extremely good piece of armor/jewelry/tools/ect. If she meets those requirements then she would be "of legal age" in the sense as we know it, despite the fact that she might be married off at age 12 simply because that sort of thing did happen. Is it weird to modern humans? Yes, it is, but then we have 12 year olds trying to get it on with each other in this day and age because they wanna grow up too fast so *shrugs*. So there you go, hope that answers your question.