Jace Ianto Winchester is the son of John Winchester and Jack Harkness. Jack is no longer in his life, but Mary has been raising the boy as her own. At Jace's six month birthday, Sam Winchester who was a demon as a result of a ritual gone wrong fed Jace demon blood That made him grow to age nine in a matter of days. A trick by Zachariah led to him aging to 17 just as fast.
***See The Road So Far for more information and plot updates***