I'm not sure how every company works, but for the most part they prefer to take on people who are in their last year (or second to last year) of college, or who they can hire full time if they so desire. While I think getting as much as experience as possible is good, I don't think it fair for someone who just started or is not even in college yet to take internship opportunities away from the people who may need them in order to graduate or get a job soon.
I used to work for a television show that once made the mistake of taking on an intern who was a Senior in Highschool. She thought she was so cool interning there, meanwhile nobody liked it after a while. She was too unexperienced as far as school knowledge, the Producers became irritated with her, and the other interns despised the fact that they had to be on the same level as her.
"It's the little things; the details, that distinguish the Barbra Streisands from the Rosalyn Kinds."~Gilmore Girls~