I would say the main reason why younger people are not getting into it is because it doesn't seem as flashy as other sports. Jibing is a huge milestone and can be very difficult, but to people outside windsurfing, its just turning around.
Also, I really started to get into windsurfing when I was 17. Before that, I could not drive myself to the beach, and my parents were too busy to drive me. Luckily I have good parents, who helped me buy my first gear. But windsurfing isn't like the other sports. Soccer, hockey, baseball, all these sports the parents can really get involved. It is like babysitting. The typical parent is too busy with work, so the kid cannot go windsurfing. It also does cost more, so a young kid has no hope to start unless his/her parents buy gear.
I think the biggest reason though is the fact that its a lifestyle. I am the only person in my school who windsurfs. Out of over 1000 kids, I am the only one who gets excited when the wind picks up, the only one who runs home, stuffs the car with gear and heads to the beach. Everyone has the perception that windsurfing is full of old guys, pink helmets, duck tape and huge boards with triangle sails. They don't know what windsurfing has become, and therefore have no interest. It is really not something you can plan, at least not here in ottawa, canada, so it takes the extra effort. You either love it or you don't.
And one more thing, if its not easy, most kids are too lazy to take it up and give it a shot. They get bored. I see why people have gone to kiting. I tried it on snow this winter, to see how easy it is... and its so rewarding so fast. 3rd time out I was pulling grabs and one handed jumps..... fast learning curve = more people doing it
My 2 cents,