I never got why GC tacked on that age limit a while back. If the kid who wins can't drive there's always a family member who can. Besides, the largest reason for that particular comp was about getting people into the store, and nobody creates a crowd like a kid and his extended family. Crowds like that never leave without buying stuff.
I think in most cases it's going to be obvious that younger guys do better at drum comps. Professional older players with a boatload of gigs and endorsements aren't getting near a contest where they have a chance of losing to a much younger person. Failing in a situation like that can lead to a number of unfortunate outcomes. So you often see the top end young guys alongside pretty good adults who play well, but aren't as seasoned as the best players their age. In that kind of situation, the young guy will always come out on top because they're usually better technically and can pull off a drum solo. Of course there's also the delusional older guys who can't play and the 7 year old wonder kids that their parents think they're going to make money off, but I think most of the participants are the former. That's at least my observation.
I was in a bunch of drum comps from the 9th grade until last year, and won and lost my fair share. But I never did the GC contest because I thought the local and regional level judging was pretty terrible. In my mind results never came out right, and I just didn't think a lot of the local judges were good enough players themselves to be judging people.
I never will forget goofing around on a pad at a local music store, when this person who I first thought was a homeless guy started asking me all these questions about single and double stroke rolls. Then he tried to do some things himself and just fell apart. A few weeks later I was shocked to see him judging a GC Drumoff. It was a total fiasco. Results were so screwed up I thought there was going to be a riot in there. People were so mad. Then there were these guys who were all upset like they now believed they were failures because this guy who couldn't even get around a real feel didn't like their playing. I just thought the whole thing was a mess.
Now to be fair, all the GC drumoffs I've seen haven't been that bad, and I once saw a state comp where the outcomes weren't perfect, but better than usual. But you have to get that far before you get guys who know what they're looking at. GC is also tied into that goofy show biz score that can be judged any kind of way, which weighs heavily towards a person who brings 50 people into the store to cheer for their awesome stick twirls. Again, just my observations.