A Japanese filmmaker is shaking Cannes film audiences to the core with a dystopian vision of her country in which old people agree to be euthanized to solve the challenge of a rapidly aging population. “Plan 75” by Japanese director and writer Chie Hayakawa is based on a very real problem. Japan is the most rapidly-aging industrial society, a trend that is causing huge economic and political problems as a dwindling number of younger people must support a growing army of the old. Close to 30 percent of Japan’s population is over 65, the majority women, and that rate is...