I wish I were younger expresses a wish for the present, so it makes perfect sense. I wish I would be younger is impossible. Would be describes a situation that would exist if another condition existed.
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AnonymousThe correct option is C. But why not others?I wish I would is almost always wrong. In this case, you have no control over how young you are, so wishing yourself to make yourself younger (I wish I would be younger) makes no sense.