Sorry, but most kids are not fully grown by age 14. It is closer to age 16 or 17. There is no denying the early commits and others being highly touted, generally speaking, are the bigger, more physically mature players. You are correct that some will get even bigger and faster through HS.....but others will not. We all know kids who were considered "huge" and practically had beards in 9th grade but were then done growing. You are totally correct that these committed kids wont stop working, but I do think some will plateau. What the percentages are for each outcome nobody knows since we are in unchartered territory with kids deciding on colleges before a single day of high school.
