The Twenty-sixth Amendment (Amendment XXVI) to the United States Constitution prohibits the states and the federal government from using age as a reason for denying the right to vote to citizens of the United States who are at least eighteen years old. It was proposed by Congress on March 23, 1971, and three-fourths of the states ratified it by July 1, 1971.

Various public officials had supported lowering the voting age during the mid-20th century, but were unable to gain the legislative momentum necessary for passing a constitutional amendment.

The drive to lower the voting age from 21 to 18 grew across the country during the 1960s, driven in part by the military draft held during the Vietnam War. The draft conscripted young men between the ages of 18 and 21 into the armed forces, primarily the U.S. Army, to serve in or support military combat operations in Vietnam. A common slogan of proponents of lowering the voting age was "old enough to fight, old enough to vote".Determined to get around inaction on the issue, congressional allies included a provision for the 18-year-old vote in a 1970 bill that extended the Voting Rights Act. The Supreme Court subsequently held in the case of Oregon v. Mitchell that Congress could not lower the voting age for state and local elections. Recognizing the confusion and costs that would be involved in maintaining separate voting rolls and elections for federal and state contests, Congress quickly proposed and the states ratified the Twenty-sixth Amendment.

Suffrage, political franchise, or simply franchise, is the right to vote in public, political elections (although the term is sometimes used for any right to vote). In some languages, and occasionally in English, the right to vote is called active suffrage, as distinct from passive suffrage, which is the right to stand for election. The combination of active and passive suffrage is sometimes called full suffrage.Suffrage is often conceived in terms of elections for representatives. However, suffrage applies equally to referendums.

In most democracies, eligible voters can vote in elections of representatives. Voting on issues by referendum may also be available. For example, in Switzerland, this is permitted at all levels of government. In the United States, some states such as California, Washington, and Wisconsin have exercised their shared sovereignty to offer citizens the opportunity to write, propose, and vote on referendums; other states and the federal government have not. Referendums in the United Kingdom are rare.

Suffrage is granted to qualifying citizens once they have reached the voting age. What constitutes a qualifying citizen depends on the government's decision. Resident non-citizens can vote in some countries, which may be restricted to citizens of closely linked countries (e.g., Commonwealth citizens and European Union citizens) or to certain offices or questions.