I also have no problem with "cadets" coming to the fire scene. They can be very helpful. As Timmy stated though, I would expect them to be trained, and supervised. I DO NOT agree with them riding on the engine though, or another first in vehicle. Leave those seats to people who know what is going on.
Here in PA, you have to be the age of 14 to join as a "junior" or "cadet". They are not allowed in the operations location of the fire scene. When they turn 16, they can throw ladders and help out with getting equipment, but by no means can they pick up tools and cut open a car or run into the building with the hose line. Must 18 to do that.
Also, you DO NOT have to have any prior training to fight a fire in PA. You can join, get a call and immediately jump on the engine and go fight the fire. Most departments put in their own rules though, where you must be trained to do these things.
Same goes with EMS. You can take the EMT-B program at the age of 15, but must be 16 upon completion and day of certification testing. This however, does not mean you can jump on the ambulance and treat patients A LONE. You must be accompanied by a "senior" EMT, someone the age of at least 18 to ride on the ambulance and give patient care in the role of an EMT. Again, some companies or departments may require the age of responders to be older, but never younger. This does not apply to "explorers" or ride-along. Those ages vary from place to place, but I think the minimum age is 14. However, I feel that still may be too young for someone to be on an ambulance in certain situations. You never know what will happen, even if you go out on a routine stubbed toe call, you may get dispatched to something much more severe and not have the chance to drop the observer off.