Most CSU campuses have a limited program for talented high school students. The trick is to not send them too young to college. Just because a child is mature, or is ready academically for a college level course, doesn't mean they should go-yet. College classes move quickly and even a well-prepared student can find the pace challenging. And don't blithely skip prerequisites unless you are certain your teen is prepared.
Courses are free for currently enrolled high school students.
Homeschoolers should use their school name to apply. The UC offers correspondence and AOL classes for high school and college. You can check local schools for independent study programs or charter homeschooling programs. Now that you are full of information, think about what you really need and want. Don't ignore the option to file an R4 and not have any outside influence.