Florida Sen. Marco Rubio called Hillary Clinton old today.

Former US Secretary of State Hillary Rodham Clinton presents her new memoir 'Hard Choices' in Berlin, Germany, 06 July 2014. EPA/MAURIZIO GAMBARINI

Ok, he didn't say exactly that. In an interview with NPR's "Morning Edition", Rubio said that Clinton is  "a 20th century candidate" who "does not offer an agenda for moving America forward in the 21st century, at least not up till now."

The point Rubio is driving at is that Clinton is politics past, not politics future -- that her ideas might work well back in the 1990s when her husband was president but that times have changed since then and the Clintons have not changed with them. Implicit in that argument, of course, is that Clinton's time has passed -- that it's time for a new generation to step forward. (Clinton would be 68 if elected president in 2016; Rubio would be 45.)

Let's put aside the age question for a minute -- since Rubio isn't (and probably won't) explicitly make that argument against Clinton if and when he (and she) run for president.  Even without considering age, Rubio has hit on what I believe is the biggest Achilles heel for Clinton in her all-but-announced candidacy: presidential races are always about the future not the past.

While Clinton will undoubtedly craft a message and a set of policy prescriptions focused on the future, the fact remains that she will be the best known non-incumbent candidate to run for president in modern memory.  The fact that Clinton has been in the national spotlight continuously since 1991 is a great advantage at one level for her; people feel like they know her as a competent and capable public figure. But, it's also a weakness when it comes to trying to package or, more accurately, re-package, Clinton as the best choice to lead the country into the future.

That seems especially true heading into 2016 given two factors unique to that race.

1) Opinions of Washington and the politicians who occupy it are at or near record lows. Being part of the first family of Democratic politics -- and being so closely associated with Washington -- is a worse thing for Clinton now than it was even in 2008.

2) The Republican field is young. Rubio is 43.  So is Ted Cruz. Paul Ryan is 44. Scott Walker is 46.  Chris Christie and Rand Paul are both 51. Only Jeb Bush, at 61, is Clinton's contemporary.

Now, being older than the likely Republican nominee isn't in and of itself a major problem for Clinton.  But, when combined with the fact that Clinton has simply been around politics for a very long time, it makes the "past vs the future" argument that much more potentially potent for Republicans.

Think back to the 2o08 race.  In both the Democratic primary and the general election, then Illinois Sen. Barack Obama effectively made the case that he, not Hillary Clinton and John McCain, represented the future.  It helped his case that he had spent just two years in Washington while McCain, first elected to the House in 1982, and Clinton were long-standing members of the D.C. establishment. It also helped that Obama was 47 years old in 2008 as compared to Clinton, who was 61 and McCain, who was 72.

Rubio is on to something with the "20th century candidate" attack. Expect to hear much more of it -- from him and other Republicans hoping to win the right to run against Clinton in two years time.