"Я не молода" is the short form of the adjective and here shows that a woman aren't young already long ago (for a long time), that is the woman means is older than you . "Я не молодая" shows that the woman not young now. But all this is very relative. "Я не молодая уже давно" = "Я не молода" = I am not young for a long time. If you take a question, that it makes no difference - "Разве я не молода?" = "Разве я не молодая?"= "Разве я не молодо выгляжу?"

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