It should just be that there's an age requirement of at least 18 years old to play in Jazz Duelist Challenge, and Jazz Multiplayer Ranked, and Jazz Clan Ladder, many of these tournaments were designed at a time when originally there weren't any adults to organize the tournament but by now many of the tournament organizers are at least old enough to be considered as legal adults.

In youth sports, there's a concept called AAU age brackets, where there's a seperate tournament for 14 and 15 year olds, for 15 and 16 year olds, for 16 and 17 year olds, and so on and so forth. Well, it does have to kind of stop there because at the age of 18 someone legally counts as an adult, but that should still at least kind of illustrate the concept of the idea of having seperate tournaments for different age groups. There's no real reason why Jazz tournaments should warrant an exception to this rule.