Thirty-nine delegates signed the U.S. Constitution on September 17, 1787.
The signing occurred at Independence Hall in Philadelphia at the end of the Constitutional Convention. Twelve states had sent representatives; Rhode Island declined to participate. Although 74 delegates were appointed and 55 attended at some point, only 39 signed the final document.
The difference between those numbers explains a common mix-up. Fifty-five delegates took part in the convention, but some departed before the final session and three of those remaining refused to sign. George Washington, president of the convention, signed first, followed by the other delegates grouped by state.
Benjamin Franklin, at age 81, was the oldest signer, while Jonathan Dayton, at age 26, was the youngest. The Constitution still required ratification by state conventions, so signing was a major step but not the same as the later state-by-state approval process.