According to National Journal writer Jonathan Rauch, Presidential candidates — like most grocery store produce — have a sell-by date. In this case, it's 14 years.
According to Rauch, no president elected since Teddy Roosevelt has ever taken more than 14 years to transition from his first major office (governor, congressman, senator, mayor) to the Executive Branch (president or vice-president). The only exception to this rule was Lyndon Johnson, who took 23 years to get from his first House of Representatives victory to becoming John F. Kennedy's VP.
2008 presidential candidates; Years in office:
John McCain = 25 years (since first elected to the US House of Representatives)
Barack Obama = 11 years (since first elected to the Illinois state Senate)
Prediction according to the Freshness Test: Obama