With each Sept. 11 anniversary that passes, a smaller and smaller portion of the American public is old enough to actually remember the events of that day first-hand. By our estimates, nearly one-fifth of the current population was born after the attacks 14 years ago -- and a quarter of Americans were too young to have any significant or meaningful memory of the day.

As of last July, the American population looked like this according to the Census Bureau.

This chart is always interesting to look at; there's a surprising amount of information contained in it. You can see the Baby Boom. You can see the birth boom among millennials and post-millennials. You can see that women often outlive men.

But what we're interested in here is breaking out how much of the population was born before or after 9/11. Since our data is already a year old, we're going to assume that last year's age-by-year figures hold for this year as well, since the increases tend to be small year-over-year. In other words, if there were an estimated 3.9 million people who were aged zero in July of 2014, we're going to assume that there were 3.9 million people aged zero this July as well. That means our assessment is necessarily low; more people were born over the last year than died.

With those caveats, we can assess how many Americans were born before and after the events of Sept. 11. Like so.

Notice that we've also colored in the first six bars of those born in the six years after 9/11. That's a rough estimate of how many people were too young to remember what happened that day. Long-term memories form prior to that, but the weight of the day probably didn't resonate with 2-year olds. So, to reiterate: This is subjective.

Using those numbers, here's our estimate of who does and doesn't remember the day.

That's 25.8 percent of the country that was six years of age or under -- or not yet born.

This is the dispassionate nature of time, of course. The rallying cry "Remember 9/11!" becomes more important with each year both because the event itself fades in memory and because the number of people who are actually able to remember it fades as well. When the Japanese attacked Pearl Harbor in December 1941, President Roosevelt called it a "day that will live in infamy." At this point, it's a day remembered by fewer than 1-in-10 Americans.

As each 9/11 anniversary passes -- somehow it has been 14 years already -- the event will become more abstract for more of the country. Already, nearly 1 out of every 5 Americans literally didn't exist when the terrorists attacked.