 FAIR: When they are in the ventilation shaft, Dylan mentions to  Elena about her "little brother". He then asks her if he is older or  younger, to which she replies younger. If Dylan refers to him as a  little brother it should have been obvious that he was younger than  her. Dylan is trying to keep Elena calm. Its possible that hes  assumed that her brother is younger in his haste and then realizes  he doesnt know for a fact, so he inquires to keep her occupied.