When DD participated last year, you could sit in on any of the competitors you wanted. She did it at Temple. I listened to several around her time and to the her friend from school. There is no limit to the number of times you can try. Her friend was a year younger and is going again this year.
The web site states:
Students from the United States must be a High School student between 14 and 18 years of age on May 31, 2008
Classical Singer Community