The Washington rally, held on a brisk, sunny day, was distinguished by the youthfulness of its organizers and many of its participants. It was led by students from Marjory Stoneman Douglas High School in Parkland, Fla., where a gunman killed 17 people last month. The survivors of the Valentine’s Day massacre have emerged as undaunted advocates for gun policy reform. A few of the speakers were as young as 11; Yolanda Renee King, the granddaughter of the Rev. Martin Luther King Jr. is 9.