Marco Rubio is a freshman Republican senator from Florida, a virtual unknown outside political circles but already a possible presidential candidate. The other day he won headlines by pronouncing himself ready to be chief executive of the greatest power on Earth.If Rubio, who turns 43 this month, were elected, he would have less than one term in the U.S. Senate. You might think of him as a drive-by senator, presumptuous in believing himself qualified for the top executive office, except that his 70 months would be more than 50 percent longer than the time Barack Obama spent in the Senate before being elected to the White House.All of which raises an important question: What does it take to be qualified to be president?Here are some people who were, by ordinary reckoning, ready to be president of the United States:-- Sen. Robert J. Dole of Kansas, a disabled World War II veteran who was in Congress for more than a third of a century.