He was born in New York City to Moya Aiken and Bill Aiken, an MTV producer. His father died of cancer in 1992, when he was only two years old. He attended Dwight Englewood School, graduating in 2008. He then went on to major in film at New York University.
Aiken made his acting debut in a commerical for the Ford Motor Company, stage debut in "A Doll's House" on Broadway and film debut in "Henry Fool". He was considered for the role of Harry Potter but lost the part to Daniel Radcliff (as he wasn't british) but went on to play intelligent 12-year-old orphan Klaus Baudelaire in Lemony Snicket's A Series of Unfortunate Events (2004).