I can't help with the majority of your questions, but I may be able to help with the age problem.
first of all, listen to how young adults talk. use the vocabulary of a young adult, or what ever age group you are writing in. don't use words above or below, depending on what kind of picture you are painting.
--
ie:
Robby looked upon his girlfriend in pain and frustration. Eyes narrowed, he stated flatly, "Hello. Listen to me, babe. We are not breaking up. Okay? Now listen to what you are saying."
Bev sighed heavily and shifted her weight as she rolled her eyes. "Robby, that's all you ever say. 'Listen to what you are saying.' Listen to your self for once! Present a good fight! You are so stupid sometimes!"
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okay. that wasn't the best of examples, but hopefully i got my point across.
another thing you could do is read a book with the age level of charractures you are writing at. that may help immensely.
hope i helped...
(edit:
about the details in the fantasy books. i
have to have details, or i become uninterested. so in my opinion, details=good.... )