The lives of Dean and his younger twin siblings, Sam and Dakota, was changed after their Mother was killed in an unexplained supernatural event. Their Father vowed revenge on whatever killed his wife, and thus, the family became hunters. When the twins were 17, and Dean 21, Dakota was killed and John, the Winchester Father, made a deal with a demon to resuscitate Dakota on the deal that she wouldn't age past the age of 17, and be stuck in a age-time-loop like situation until she died.