An individual who is going through puberty.
Synonym: See Thesaurus:pubescent
At or just after the age of puberty.
Covered with down or fine hairs.
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.”