AS Bill Clinton was the first baby boomer president, Barack Obama could be the first Generation X president. Or, depending on how you figure it, Mr. Obama, born in 1961, could be the third boomer in chief, following Presidents Clinton and Bush. In theory, the candidate Obama belongs in the boom, defined by the Census Bureau as births during the years 1946 to 1964… The generation-spotter Jonathan Pontell, on the other hand, argues the boom began in 1942 and ended in 1953. He places Mr. Obama in “Generation Jones,” a term Mr. Pontell coined to characterize those born during the...