When must all men register for the Selective Service?

Why this is the answer

Either official answer is correct: at age eighteen, or between eighteen and twenty-six. Both describe the same requirement, which applies to men between 18 and 26 years old. The most important thing to understand is how little registration involves. There is no draft today, and registering is not the same as serving. Registering simply tells the government that a man is available to serve in the U.S. Armed Forces, and it can be done at a post office or online. The system has a long history. During the Civil War, President Lincoln’s attempt to draft men to fight led to anger and riots. In 1917 Congress passed the Selective Service Act, handing President Woodrow Wilson the power to enlarge the military temporarily during World War I. In 1940 President Franklin Roosevelt signed the Selective Training and Service Act, the first draft in peacetime and the beginning of the Selective Service System that exists today. The country needed the draft once more for the Korean War, and then again for the Vietnam War. In the Constitution, Article I gives Congress the power to raise and support armies. The topic also connects to the Oath of Allegiance from earlier in this section, where new citizens promise to bear arms or perform noncombatant service when the law requires it. Several wrong answers are tempting. One is twenty-one, which sounds like an adult milestone but has nothing to do with this rule. Another is “only when there is a war,” which confuses registering with being called up; the requirement applies right now, with no draft in place. A third, “when they become citizens,” misses that the rule is set by age: the lessons say all men between 18 and 26 must register. Saying “at eighteen” gives the shortest complete answer, and adding “up to twenty-six” shows you know the full window.