The story
Blend of 'tween + teenager. Attested as early as the 1950s, but popularized in the late 20th or early 21st century.
Etymology from Wiktionary, CC BY-SA 4.0.
tweenager
Today it means: A young person who is not yet a teenager (being less than thirteen years old), but starting to act like one: generally ten to twelve years old. Full definition →