Green’s Dictionary of Slang

a pre-teenager, poss. more sophisticated than their peers.

1999		Indep. Rev. 8 Nov. 4: The presence of the former minister went down well with the ‘tweenies’ (the school is for girls up to 11 years old).
1999		Observer Rev. 1 Aug. 4: It is no longer enough for the more sophisticated pre-teen [...] the ‘tween’, to ransack their mother’s make-up bag.
2011		Dly Record (Morristown, NJ) 5 June 39: A surf-board-carrying tween named Dude (‘a way cool guy’).
2013		T. Robinson Hard Bounce [ebook] [W]e could easily police a few dozen skinny tweens.
2024		J. Haidt Anxious Generation 8: Kids who are 10 to 12 are between childhood and adolescence, and are often called tweens for that reason.