User blog comment:TheCarrotSaysYumYum/FT0 2 - Game of Truth, Review./@comment-25111584-20140820064643/@comment-31.61.131.198-20140821065219

Gray can't be 13, unless it's alternative universe with different timeline or he is already a Fairy Tail mage. It was shown in flashbacks that he was in FT before Erza, and IIRC she was 11 when she came there, so he would be at most 10. But there's also the material from FT special (not sure about level of canonicity of this one lol) where Lucy asks Mira when everyone came to the guild and she said Gray came to FT 10 years ago, which would probably make him 8 (or 9 at most).