A child who has not yet reached puberty.
Of or relating to preadolescence.
A young person who is not yet a teenager (being less than thirteen years old), but starting to act like one: generally eight to twelve years old.
1994, Mary Manz Simon, How to Parent Your “Tweenager”, page 6
Your eight to twelve year old is in the gray area between early childhood and the teen years. He is a child in the middle—a “tweenager.”