1 immature, undeveloped, underdeveloped, unfledged, untimely, unready, early, unripe, green:
Much premature fruit has been blown down in the winds.
2 hasty, untimely, ill-timed, too early, too soon, beforehand, unseasonable, overhasty, impulsive, inopportune:
The post may be slow, but still I think September a bit premature to send Christmas cards. The discovery that money was missing accounts for his premature departure.