He showed talent as the kidnapped Indian "Red Chief" in a segment of the film O. Henry's Full House (1952) and another kidnap victim as the son of scientist Gene Barry in The Atomic City (1952).
He also appeared in other film styles such as the film noir thriller Jeopardy (1953) with Barbara Stanwyck, the horse opera drama Arena (1953) with Gig Young, and the comedies Mister Scoutmaster (1953) with Clifton Webb and Ricochet Romance (1954) with Marjorie Main.
In 1953-54, Aaker was among the many child actors who auditioned for the role of "Jeff Miller" on the original 1954 Lassie series, which later aired as Jeff's Collie. That role went to Tommy Rettig. Two weeks later, Aaker won the role of "Rusty" on The Adventures of Rin-Tin-Tin, co-starring James Brown (1920–1992) as Lieutenant Ripley "Rip" Masters. Aaker and Rettig were friends, and both shows were popular with audiences. The two actors and the two star dogs appeared together in a photograph used on the cover of the July 2, 1955 issue of TV Guide.
In the late 1960s, Aaker was married to Sharon Ann Hamilton for two years. He has resided in Mammoth Lakes, California, for many years and is a special-needs ski instructor.
In 2005, Aaker won the Golden Boot Award as Kids of the West.
