A little research yielded the following information:
In California, “No student whose nineteenth (19) birthday is attained prior to June 15 of the prior school year shall participate or practice on any CIF team. A student whose 19th birthday is on June 14 or before is ineligible.”
Regarding whether 8th grade graduation starts the clock ticking on high school athletics, it would appear that enrollment in 9th grade is the official start: “A student who first enters the ninth grade of any school following the student’s completion of the eighth grade in any school may be eligible for athletic competition during a maximum period of time that is not to exceed eight consecutive semesters following the initial enrollment in the ninth grade of any school. Said eligibility must be used during the student’s first eight consecutive semesters of enrollment at that school or any other school.”
These are taken from http://www.cifcs.org/parents/protect-student-eligibility