Counterculture is mainly driven by the youth because

The 1960s were a period when long-held values and norms of behavior seemed to break down, particularly among the young. Many college-age men and women became political activists and were the driving force behind the civil rights and antiwar movements.

Counterculture is mainly driven by youth because of inertia. Because older people have already fully developed brains, they resist change. They have learned to accept the world for how it is. The youth are still developing and are open to change. They are at a stage of extreme curiosity, which provoke the question of why things are the way they are.