tweenager

Blend of between +‎ teenager. Attested as early as the 1950s, but popularized in the late 20th or early 21st century.

tweenager (plural tweenagers)

1. 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.”

• See Thesaurus:preteen

• tween

• “tweenager”, in Collins English Dictionary.
• Michael Quinnion (2001 April 28) “Tweenager”, in World Wide Words