President Trump and first lady Melania Trump participate in a moment of silence in remembrance of those who died on Sept. 11, 2001, at the White House on Sept. 11, 2017. (Melina Mara/The Washington Post)

It struck me the other day that at some point I would have to explain the terrorist attacks of Sept. 11, 2001, to my son. He was born in January this year, so there’s a lot he needs to learn and not much I can teach him over the short term. But at some point I’ll need to explain why there are lights in the sky every September in Lower Manhattan and what happened that day to his country and to his family.

He’s got to learn about everything, of course, but this particular thing is different: one of the most emotional days of his parents’ lives and one of the most significant days in American history. How do you convey that? How do you explain the context of events that played out over a few hours one day that jolted the country in a new direction?

That’s a question that a lot of people have tried to answer over the past 16 years. Schools have explored curriculums on the subject. The 9/11 Museum in New York has its own lesson plans broken out by each group that might need them: K-2, 3-5, 6-8, 9-12. Kids graduating high school this year will have no memory of the events of that day; nearly all were not even born by then.

In fact, more than a fifth of the country was born after 9/11, according to 2016 data from the Census Bureau. (Data for this year isn’t yet available, so the figures below use the most recent estimates.) That varies by state, with the Northeast generally having a lower percentage of the population falling into that category. (In other words, it skews older.)

By state:

The states with the largest percentage of residents who were not alive for 9/11:

  • Utah, 28.6 percent
  • Texas, 24.8
  • Idaho, 24.5
  • Alaska, 23.9
  • Nebraska, 23.5

Utah’s population is younger than most, as evidenced by that data point. But that a quarter of Texans were born after the events of 9/11 is remarkable.

The smallest:

  • Rhode Island, 18.5 percent
  • New Hampshire, 18.3
  • Maine, 17.9
  • Vermont, 17.8
  • District of Columbia, 17

In New York, where my son Thomas was born, the figure is just shy of 20 percent.

Since this is 2016 data, all of those figures are actually larger for this year, as people are born and older Americans pass away.

That a new generation of Americans needs to learn our history isn’t a new phenomenon, of course. It’s just that 9/11 still seems so near and looms so important, amplified by its overlap with the early days of the Internet. By the time Thomas is ready to learn, I’d estimate, the process of doing so will be even better defined.

After all, it’s likely to be that a full quarter of the country will only know about the attacks secondhand by then. The memory of that day will continue to recede, for better and for worse.