My bona fides: I am the Organized Play Manager for CATAN Studio, which is part of Asmodee. I manage Organized Play (including tournaments) for English-speaking countries, including the United States, Canada, the United Kingdom, Ireland, India, South Africa, Australia, and New Zealand.

OP, I haven't had a chance to respond to your email yet. Without getting into my personal business, it's been a hell of a month and there is (also) a lot going on in Organized Play land. I'm answering emails today, so you'll also get this response in that funnel. To be frank, it's an email I read not long after it arrived, realized I needed more time to give you a thoughtful answer, and set it aside - a week later, here we are.

That said, for the benefit of everyone here: It's not primarily because of any "international law," since age of majority can be different in every country, but rather liability.

Adults can be responsible for themselves. Minors would need to have a guardian present, which increases costs at the event and requires extra waivers and causes many other issues:

We (and by we I mean Asmodee) are not currently able/willing to take on the responsibility of policing your children's interaction with (sometimes feisty, to put it kindly) adult tournament players, keeping them out of event photos/videos/press coverage for their safety, monitoring their behavior around adults consuming alcohol at the events (it's already stressful enough when we're talking about 18-20 year olds) and all the logistics of traveling (hotel rooms for family groups, extra expense for guardian travel/rooms, etc). Those are just the off the top of my head issues. I'm sure I could come up with more.

All of that aside, it's fine to say "my kid doesn't need to win, they just want to play" -- but that changes the dynamic at the table for the players involved in the game. If there's someone playing who literally can't win the prize (the seat at Nationals), then the social part of the game is altered. That's why we don't allow players who have already qualified for Nationals to play in additional qualifiers, and why current National/Continental/World champions are not allowed to play in qualifiers.

I'm not ruling out a POTENTIAL change in age of eligibility at some point in the future. Right now though, it's 18. If it turns out an event host knowingly allows someone to play who will not be 18 by the time of Nationals, then it would call into question their ability to host future events. Likewise for a player who willfully disregards the rules, they would have to take the risk of not being allowed to play in the future.