Kerry, Sanders and Biden are all 75 or older. Warren is 69. Yet my concern about their potential candidacies have nothing to do directly with their chronological age. None of the four shows any signs of age-related issues, and they are all perfectly capable of significant continued service. But in part because of their age and their long resumes, they will inevitably be identified with an era that seems increasingly irrelevant.

Whatever else it will say about him, history will see Donald Trump as a marker in time that divides what came before him from what will come after him. He is a ridiculous figure, especially as president, but the fact that he was elected anyway demonstrates the bankruptcy of that previous era and the repudiation of an approach to politics that had grown stale and unresponsive.

Hillary Clinton -- former First Lady, former senator and secretary of state, a national figure for 30 years and also clearly well-qualified to be president -- embodied that previous era, which is largely why she lost. She is also a reminder that nobody is owed the presidency or a nomination.

Through incompetence, ignorance and no small degree of malevolence, Trump is hastening the destruction of that previous world. Alliances, networks and understandings that had stood for decades are falling apart, quickly, but the truth is that all that was fated by time to crumble anyway. Wiser leadership would attempt to manage that decline while building replacements, but for the moment that is not the leadership that we have given ourselves.

Today's challenges -- climate change, unsustainable economic disparity, technological disruption, the rationalization of a dysfunctional health-care system, the restraint and redirection of nationalist passions, the growing despair and addiction in those parts of America left behind by change -- these are the troubles that must now be addressed. Voters will want and deserve fresh voices and insight into tackling them.

Given the news trend, it’s also dangerous for Democrats to assume that the 72-year-old Trump will even be the GOP candidate in 2020. To cite an example, Biden might defeat Trump handily and would very much enjoy doing so, but a race between Biden and, say, Nikki Haley might produce a very different outcome, for all of the reasons outlined above.

It's time to move forward, not reach back.