Is there a high school varsity football age limit or minimum?

There's no national age limit for high school varsity football. Eligibility is determined by the individual state's high school athletic association and their rules, which often center around grade level (typically grades 9-12) and age relative to the school year (sometimes with a cutoff date). A student might be too old to play if they've aged out of the permissible grade level. There's usually not a minimum age, as long as the student is enrolled in the appropriate grade.