Sure, I’ll take a stab at translating. Two guesses, which aren’t mutually exclusive: 1) The agent thinks the narrative voice sounds too old for a story told by a tween. Perhaps the words are too fancy for a kid, or the sentence structure too complex, or the insights too sophisticated. Give each of those a look. 2) The agent thinks some of the things mentioned in the narrative were things that your POV character could not know. Make sure your first person narrator only mentions things she can know first-hand.