EXACTLY 100 years after he became, at the age of 42, the youngest man ever to be sworn in as president of the United States, interest in Theodore Roosevelt is experiencing an unprecedented revival. Former Speaker of the House Newt Gingrich loves Teddy; so does President Bush’s campaign strategist, Karl Rove. Across the aisle, House Minority Leader Richard Gephardt sings his praises, while former president Bill Clinton, tutored by Washington Post columnist and T.R. aficionado E. J. Dionne, sees him as a model for the exercise of energetic executive power.