BenP, the SCISA rule is that 7th and 8th graders are elegible to play certain varsity sports, soccer being one of them. Other varsity sports, swimming being one of them, the lower age limit goes down. SCISA also has an 8 semester rule for elegibility, meaning, when a player enters the 9th grade, he has 8 consectutive semesters of varsity participation. It's available in the bluebook at the SCISA web page. So for SCISA, if the talent level and physical maturity are appropriate, there are 6 possible years for varsity soccer participation. There was a 7th grader on Pinewood's Championship team from last year.
I have heard about the contact sport ruling for the SCHSL, meaning no varsity play until 9th grade. Not being part of the SCHSL, I haven't had the need to do the research.
