Policy can vary widely. One cc in my state has welcomed homeschoolers as young as ten with open arms. Another has had a strict age 16 only policy (unwritten, but "everyone" knows about it) which they enforce by making the younger candidates jump through hoops- an interview, a placement test, another interview, bizarre questions about how they will "make" older students feel comfortable with their presence and how they will deal with returning war vets who may have flashbacks in the classroom. All in all, it's a maze designed to encourage kids to give up and go home until they are 16.
Informal mentorship with a prof is a good way to establish relations with a school and allow your child to show his stuff. Then you have an advocate who will go to bat for him.
