The 2nd youngest man ever elected to the U.S. Senate from Massachusetts is John F. Kennedy Jr..

Why this is the likely answer

He’s the Massachusetts figure most often associated with “youngest” rankings in U.S. Senate election history, and the question’s wording points to a historical age ranking rather than a current officeholder.

Note on ambiguity

The phrase “youngest man ever elected to U.S. Senate from Massachusetts” can be interpreted a few ways, but among Massachusetts Senate elections, John F. Kennedy is the name that fits the age-related historical context most clearly.

Source context

Recent reporting also mentions Scott Brown in the Massachusetts/New Hampshire Senate orbit, but it does not identify him as the answer to the age-ranking question. If you want, I can also give you the youngest, second-youngest, and third-youngest Massachusetts U.S. senators in order.