A 21-year-old man was killed in a car accident on I-84 in Danbury early Friday morning.  State Police say Aaron Thomas Young, of Colchester, was traveling east when he dove into the left shoulder and through the wire rope guardrail.  His vehicle came to rest more than 50-feet down an embankment near exit 7 around 1:30am.  Thomas was pronounced dead at the scene, police said. His passenger was transported to Danbury Hospital for treatment of minor injuries.  Any witness to the crash is asked to contact State Police Troop A.