@my3girls I believe most or all allow some HS students to take classes. Some HS do not take the credits because they can be used to pad weighted HS GPAs. I took CC classes as a HS senior back in the 1980s.
There was a successful age-discrimination lawsuit several (many?) years ago. But, there are different implementations. I have seen one CC that says they only allow dual-enrollment for HS juniors and seniors. Ours has free dual-enrollment to students from the summer after 8th grade through HS graduation. There is a petition process here for even younger students (mostly homeschoolers). The part about being free (except books, parking, and activity fees) probably differs widely depending on funding mechanisms for dual-enrollment.