An unacknowledged winner of 2016 has been John McConnell, a veteran Republican speechwriter who served in the George W. Bush White House. According to Jonathan Rauch, McConnell outlined the following rule in the early 2000s: "No one gets elected president who needs longer than 14 years to get from his or her first gubernatorial or Senate victory to either the presidency or the vice presidency." As Jeffrey Anderson noted in The Weekly Standard last year, that 14-year rule holds for every presidential election since 1860. Thus, once a presidential aspirant gets elected to the Senate or the governor's mansion, he or she has 14 years to make it to a winning presidential ticket (either as VP or president) if he or she hopes to be president at some point. Obama had 4 years between being elected to the Senate and winning the White House, and George W. Bush and Bill Clinton had 6 and 14 years, respectively, between winning their first gubernatorial race and the presidency. George HW Bush had never been elected to the Senate or a governorship, so the 14-year rule did not apply to him.
There's obviously no metaphysical reason why this 14-year rule has to be the case, but Rauch and Anderson have some interesting reflections on why it is a trend. It seems to suggest the American people's preference that presidents have some experience but also represent something politically fresh.
In 2016, Donald Trump (never previously elected to any government office) faced off against Hillary Clinton, who was first elected to the Senate in 2000. Secretary Clinton was thus 2 years past the 14-year rule. (Interestingly for Democrats, Joe Biden would also have been well past the 14-year rule; first elected to the Senate in 1972, it took him 36 years to make it to the vice-presidency. Bernie Sanders, however, would not have run afoul of the 14-year rule; he was first elected to the Senate in 2006. Nor would Hillary Clinton have been past the 14-year rule if she had been the Democratic nominee in 2008 or if Obama had picked her to be vice-president in 2008.)
When Donald Trump beat Hillary Clinton, the 14-year rule won again.