Gregory Piper was 13 when he first appeared in Line of Duty series one as Ryan Pilkington, a teenager recruited into the OCG and tasked with couriering untraceable burner phones (he also, as we were reminded in series 6, tried to cut off DS Arnott's fingers with bolt croppers). In series six, Piper reprises his role, except now he's a constable at Hillside station, brought on by DCI Joanne Davidson.
Image Source: BBC Pictures