According to the dictionary a tween are people who are to young to be a teen yet to old to be a child. If you are either 11 or 12, you are a tween.
Demographics:
Infant (0-1)
Toddler (1-2)
Pre-schooler (3-5)
Child (6-10)
Tween (tweenager)(11-12)
Teen (13-19)
Young adult (20-26)