SACRAMENTO, Calif. — When Nia MooreWeathers first heard about California’s sweeping plan to shutter its youth prisons and transform the state’s juvenile justice system, she was filled with hope. As a community organizer for the Sacramento nonprofit Youth Forward, she saw it as a long-overdue opportunity to reimagine how incarcerated youth are housed and treated. Juvenile offenders would serve their time closer to home, in settings that emphasized mental health care and other supportive services. But before long, MooreWeathers and other advocates across the state say, they realized it would tak...