High school students are welcome. There is a "junior" league intended for our 1st year students who have little experience with algorithms. A number of problems there are appropriate for high school students as well, especially those who have had a bit of practice solving problems of this nature.
While we don't discourage even younger students from coming, I would strongly suggest they look closely at the type of problems we address before deciding if they want to come. Last year's contest URL is here: