I think the origins of the anti-nuclear movement are generally associated with the flower-power/Summer of Love/anti-war/early environmental movements of the late 60s/early 70s, which are generally associated with the baby boomer generation.
Diablo Canyon began construction in 1968. The oldest baby boomer would have been 22. Seems reasonable to conclude that the decision to build the plant predates them.
EDIT: Confused by the downvote—does someone disagree on the timeline?
"The initial objective of the movement was nuclear disarmament, though since the late 1960s opposition has included the use of nuclear power. Many anti-nuclear groups oppose both nuclear power and nuclear weapons. The formation of green parties in the 1970s and 1980s was often a direct result of anti-nuclear politics."
