By 'right', one assumes there is justness or a given rightness to an ideal or concept. Neither of which applies, and also falls under a similar principle to 'fairness.'
It's not a 'cop out', it's the truth. For whatever reason, the current age divisions are the way they are. But if they were to change, there will be a reason for doing so, and the reasoning against it ALSO can't be substantiated by 'fairness.' Fair involves freedom from self-interest, and it would be in the self-interest of 12 year olds to say they SHOULD go to Worlds.