A child between the ages of roughly 8 and 12, not quite a teenager but no longer a little kid.
A modern portmanteau combining 'between' and 'teen,' created in the 1980s as marketing terminology to describe a specific age group and consumer demographic.
Marketers literally invented the word 'tween' to sell products to kids aged 8-12—it's one of the clearest examples of how business shapes the language we use to describe human life stages.
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