One hundred years ago on Aug. 1, Arthur Eldred, a 17-year-old Boy Scout from Long Island, became the first person to earn the Eagle Scout rank. Eldred, tall, quiet and with a shock of dark hair, had joined scouting largely at the behest of his widowed mother, who hoped it would give some structure to his life. Yet as Eagle Scouts would continue to do throughout the next century, Eldred caught the scouting world by surprise. He was the first of an extraordinary new cohort of young men who were to prove very different from the classic 13-year-old Boy Scout in short pants.

Eldred's...