The sport really appeals to all ages, its just kids can be easily presented to the sport more than a teenager or adult would. Kids are looking for something to get into and normally go for the thing that makes the most noise or does something cooler than anything else. Thats why you see kids into Iron Man, he blows stuff up and flies around, monster trucks crush cars and are loud. A good example is the truck in my avator, Hammerhead from Twisted Metal,, everyone wanted to play it because it was a monster truck, and it made things go boom.
People in general like big trucks, weither they are monster or not. I have friends and co-workers ages from 18-38 that absolutely love my fathers truck because its lifted and has mud tires, and when I bring in monster truck photos, they are so amazed a tire is that large, or the fact the truck is driving over a bus, or driving across a river.
It is a sport, and hopefully some more promoters start to see it that way again and break away from "kids are the future of the sport", that statement isn't neccessarily true. Kids are the future, mostly because we need that kid to make a monster truck when he becomes an adult so the sport can live on.