Who is the youngest congressman ever to be inducted?

In the 19th century, several state legislatures elected Senators in their late twenties despite the Constitutional minimum age of 30, such as Henry Clay, who was sworn into office at age 29, and John Henry Eaton, the youngest US Senator in history, who took his oath of office when 28 years, 4 months, and 29 days old.

John Henry Eaton, the youngest US Senator in history, who took his oath of office