“I do not think you will count, Harry: You are underage and unqualified. Voldemort would never have expected a sixteen-year-old to reach this place: I think it unlikely that your powers will register compared to mine.”
These words did nothing to raise Harry’s morale; perhaps Dumbledore knew it, for he added, “Voldemort’s mistake, Harry, Voldemort’s mistake . . . Age is foolish and forgetful when it underestimates youth.”

Harry Potter and the Half-Blood Prince – J.K. Rowling