Across the country, Americans took a moment on Wednesday morning to remember the terrorist attacks of Sept. 11, 2001. Or, for about a third of Americans, the moment was one of observation for the thousands of lives lost.

An observation, not a remembrance, because those Americans were either not born when the attacks occurred or were too young to remember them.

This is how the world works, of course. By 1983, 40 percent of Americans were either not born or too young (that is, 5 or under) to remember the assassination of John F. Kennedy. Time passes; we get older; the events that defined our lives are not ones that younger people experienced.

This event, though, was defining for many Americans. It defined American foreign policy for decades to come, affecting even those who hadn’t experienced it. As The Post reported in 2021, half of Americans killed serving in Afghanistan weren’t adults when 9/11 occurred. Five of those killed in the attack at the Kabul airport as the United States pulled out of Afghanistan in 2021 were only 20 years old when they died.

The most recent data from the Census Bureau indicate that about 30 percent of U.S. residents were born after 2001. An additional 7 percent were 5 years old or younger. That’s 37 percent of the population that was at least too young at the time of the attacks to be cognizant of what unfolded.

There’s another striking aspect to how long ago the attacks took place. Those who lost their lives that day ranged in age from 2 to 85. On average, they were 40 years old. Today, had they survived, they’d be an average of 63 years old. That youngest victim — Christine Lee Hanson, a passenger on United Flight 175 — would be 25. Only a handful of victims would still be younger than the average age that day.

Again, this is how the world works. Tragedy fades or is usurped by new tragedies. We still observe memorials for the Triangle Shirtwaist Fire, but for several reasons, the annual ceremony in Lower Manhattan doesn’t attract the crowd seen at the site of the World Trade Center attacks.

The mantra that emerged after Sept. 11 was “never forget.” By the late 2030s, the Census Bureau projects, most Americans will be too young to have remembered the attack in the first place.