Madlax just had a huge start on it since they've been around for awhile and picked up on it early. BLC used to be purely recreational but have become much more serious in travel. NL became more focused on travel when the Mitchell's took over and their popularity has translated into attracting better players who are most likely older.
It's also not limited to private school. If a public parent starts his kid late to first grade, as in the case of summer birthdays, The kid stays in that grade and there are plenty of those examples on the club and high school teams. This is why 19 years old publics can play high school sports. The problem comes when those older kids are then moved to private and held back. Then they become almost 2 years older than USL rules allow.
I thought I saw somewhere where USL caved a little and are allowing 5 exceptions per team for their sanctioned tournaments.
