As the mom of a boarder who is very small for his age, I find the use of age divisions rather than skill set divisions to be detrimental to not only my own son's snowboarding, but ultimately to US Snowboarding.
If you look at any of the other "judged" individual sports like gymnastics, diving, or figure skating, they use skill levels (where either there are required elements and/or maximum difficulty caps) rather than age (or sometimes combined with age to breakdown a level in further). Doing so allows the lathletes to develop at their own pace, feel successful and not get discouraged especially if they come to the sport later or are later bloomers physically.
The reason I say it could end up hurting US Snowboarding in the long run is that there may very well be great potential snowboarders who, because they must compete with kids their age but who might have years more experience or be far bigger physically at a particular time (allowing for my speed and height off of jumps etc) that these athletes end up discouraged and dropping out when they may just have needed more time to get to the same spot.
Additionally leveling allows the athletes and their coaches to keep them to moving along, using the correct techniques while allowing them to develop at their own pace... Instead of having kids throw skills they aren't ready for just because the leadersin their age group are already doing it.
In my opinion, for slopestyle and rail jam snowboarding, the current system should be reexamined and compared to other "like" sports rather than assuming the groups for alpine and bx apply across the board.
I'm sure this question has come up a lot and I'm sure that there are plenty of people in this same boat. Size doesn't have to be a correlation to the skill set that a rider has. Riders will all develop at different paces for a world of different reasons. The "leaders" in your sons age group may have much more experience riding or have a full time coach working with them. As I said there are so many factors in the equation of a riders development. Comparing snowboarding to any of the more traditional sports is like apples and oranges. They are far too different.
US Snowboarding has been and remains the most dominant snowboard across the globe. The US puts out far more podium contenders in almost all facets of snowboarding than any other country. Freestyle athletes often tend to be smaller than sbx athletes and that can often be an advantage. Finding a coach that is able to properly develop your athlete is very important, if done correctly physical stature doesn't have to be an issue at all. You are correct that kids should not be going out and "throwing" tricks that they aren't ready for. However, it is this competitive nature that drives all sports. The athletes continue to push each other at all levels as each one learns a new skill. Just make sure that skills aren't skipped in the process of trying to learn one.
As for the leveling, there is somewhat a system in place. USASA regional events are the low ones on the totem pole. Nationals is directly above that. Kids that continue to do well at nationals usually go on to compete in Revolution Tours. SBX riders will race Norams after that and then the big jump up to World Cups. After Revo Tours for freestyle riders there are a plethora of other events from Burton Ams, Gatorade Free Flow Tour, Forum Youngbloods, etc. From there they move on to Grand Prix's, and so on up the ladder. It was interesting to hear your thoughts. Hope this was helpful