That is emphasizing that you are not young but rather something else. It's like playing a game with several people who each have a pre-determined quality and you need to find out who is tagged with which. Then you point at someone and guess, "you are the young" - but no "I am not the young one".
Your version is grammatically not wrong, but it is not really a good translation of the english sentence.
Thank you ! I always have a problem with the order of the words : I can't figure out I'm emphasizing when I make a sentence, or what others will understand if I say the right words in the wrong order.... but I guess it's just a matter of practice !
Thanks again for answering me !