The phrase "most pre-adult" refers to the stage before adolescence, usually between the ages of nine and twelve. Synonyms for this term could include "pre-teen," "tween," or "prepubescent." These terms all capture the idea of being young but not quite a teenager yet. Additionally, you could use phrases such as "in the pre-adolescent stage," "approaching puberty," or "in the midst of early adolescence" to describe this developmental phase. It's important to remember that each individual may hit milestones like puberty at different times, so while these terms can be useful, they are not always strictly defined by age.