(Informally) a child approaching the age of 13?

• Youngster is quaint on horse, that girl heading off
• Fifth grader, often
• Many a YA reader
• One of Shakespeare's sisters put up with precious pre-adolescent

Wiktionary Word definitions in Wiktionary
n. A young human being that is not yet technically a teenager (being less than thirteen years old), but starting to act like one—generally eight to twelve years old.