preteenager
noun
pre·teen·ag·er (ˌ)prē-ˈtē-nā-jər 
: preteen
Example Sentences
Recent Examples on the Web Campanelli’s office represented a 12-year-old boy who was ordered confined in September 2018 by a Cook County judge, despite an ordinance passed by the Cook County Board of Commissioners that barred the custodial detention of preteenagers. 
Emily Hoerner, chicagotribune.com, 8 Nov. 2019
Emily Hoerner, chicagotribune.com, 8 Nov. 2019 Word History
First Known Use
1948, in the meaning defined above