Does anyone remember the boy that was half demon half human? He got away .And Sam and Dean never found him .

Jesse teleported away and disappeared. He used his power to ensure that no one could find him so that the demons couldn't end up using him to win the Apocalypse, and the Engel oder the Winchesters wouldn't have to kill him to prevent that from happening. The poster in his room suggested that he may have fled to Australia.

They did not find him again the kid vanished after going to his room.