Midshipmen place 2,996 mini American flags on Stribling walk, to represent those lost on 9-11, at the U.S. Naval Academy on September, 10, 2016 in Annapolis, MD.
(Bill O'Leary/The Washington Post)

There are about 4.2 million people living in America who were born in the year 2001. According to the most recent Census Bureau estimates, from 2015, about 61.2 million Americans were born in or since 2001 — just shy of a fifth of the country.

We used this data last year to figure out how many Americans were too young to remember the terror attacks first-hand. When you throw in people who were five or younger, we ended up with an estimate that about a quarter of the country fit into that category. Per the 2015 population estimates, the figure is about 26.9 percent.

Never forget? They had nothing to remember in the first place.

To remember 9/11 first-hand — assuming a six-year-old would — you have to be at least 20. But even among those adults, memories of the events of that day are surprisingly spotty.

That’s according to data from Pew Research released this week. Pew surveyed American adults 18 and older, asking a number of questions about the attacks of that day. Included among them were whether or not the respondent remembered where they were when they learned of the attacks and if they remembered the year the attacks occurred. Most people remembered the former and remembered the latter correctly — but not all.

Over 90 percent of all adults remembered where they were when the attacks occurred. The figure for those under the age of 30 is lower, which makes sense due to the fact that some fraction of the group hadn’t yet been potty-trained. Even among those aged 65 and up, the number neared 9-in-10.

Asked what year the attacks occurred, the numbers were a bit worse. Only a bit over two-thirds of adults correctly identified the attacks as having occurred in 2001, a figure that decreased as respondents got older. Those 65 or older only got the year right about half the time — but, then, they’ve had a few more years to keep track of. And, for that matter, a few more national tragedies.

Interestingly, that number isn’t much different than the figures from 2006, when Pew also asked the question. Seventy percent of all Americans answered correctly, and even fewer of those 65 and older got it right.

Pew didn’t track the number of people who literally don’t remember the attacks themselves happening, but it’s safe to assume that the figure would be very, very small. Which isn’t really the point of the “never forget” mantra, of course. The point is to remember the lives that were lost and the spirit of unity that followed.

In a poll earlier this year, Gallup got at that second question, finding that fewer adults identified themselves as “extremely proud” to be American than at any point since the attacks.

The number among youngest Americans, those least likely to have been alive on that day, were lower than any other group.