Foster was born in Boston, Massachusetts, the son of restaurant owners Gillian Kirwan and Steven Foster. He has described his parents as "free-spirited, Vietnam-protesting hippies". He has a younger brother, Jon, who is also an actor. When Foster was four years old, his family relocated to Fairfield, Iowa, after their Boston home was broken into by robbers while they were present.\n', '
Foster was raised Jewish, and had a Bar Mitzvah ceremony. His paternal grandparents were Celia (Segal) and Abraham Foster, who was a prominent judge and politician in Boston; their families emigrated from Russia. While living in Fairfield, he attended the Maharishi University of Management.\n', '
Foster began working as an actor when he was sixteen years old. In 1996 and 1997, he starred in the Disney Channel television series Flash Forward.\n', '