You!
That's who. These clinics are designed to introduce cyclocross to those who've never tried it and for those who have raced before and want to get their legs and lungs into shape for the upcoming season.
Age?
Yes! Any age will do. (You don't even have to be mature. In fact, it probably helps if you're not.)
Season
In this area, cyclocross season runs from mid-September to early December. Nationally and internationally it will go all the way into February. It's run in all kinds of weather, from sunny, dry and warm, to rainy and sloppy, to bitter cold and snowy.
Course
For the uninitiated it looks a bit wacky, with riders pedaling over uneven terrain (often park lawn areas) on twisty courses, running up hills either pushing their bikes or with the bikes slung over their shoulders, and hopping over barriers. It's a great way to stay in shape in the fall, do something a little crazy, and share the challenge of a unique form of cycling.
Bikes
You can ride just about anything durable, though there are bikes specifically designed and sold for just this kind of event...cyclocross bikes (what else?). Many people ride their mountain bikes as well. It doesn't make any difference what you ride for these clinics. Bring what you have and just get into the fun spirit of it all.
Intensity Levels?
It's serious for those who make it so, and not so serious for those who don't. All are welcome and supported.