Moorer was born in Santa Monica, California. His parents are George Edward Moorer, a retired salesman and entrepreneur, and Carrol Rothe Moorer, a nurse. When he was 11 years old, his family moved to the Monterey Peninsula. His mother acted in amateur performances in the San Fernando Valley, and Stephen got an early taste of performing in community theatre. His first principal role was Miles in "The Innocents" (based on "The Turn of the Screw"), with The Rafters Theatre Guild. Moorer attended the Carmel, California middle and high schools, becoming actively involved in the drama program, acting in and producing shows. In his senior year, he played a criminal mastermind in "Wait Until Dark". From the age of 11 to 17, Moorer also studied theatre at Carmel's Children's Experimental Theatre.