13-year-old boy , who is a member of the notorious wood green gang , pleaded guilty to stabbing making him one of britain 's youngest ever murderers .
the teenager , who can not be named for legal reasons , pulled a knife from his rucksack and stabbed mr barry , a builder , as he got into a lift following a row .
the old bailey heard the boy became enraged when mr barry would not let the boys into the flat if the occupants were n't answering the buzzer .