Most of the photos are slightly faded. The hairlines fuller. Some feature braces. Old friends. Sorority squats and college sweethearts. Caps and gowns. Laments about skinny jeans and other long lost trends.

This week, Instagram stories the world over have been awash with nostalgic snapshots of youthful idealism — there have been at least 3.6 million shares, according a representative for Meta — as people post photos of themselves based on the prompt: “Everyone tap in. Let’s see you at 21.”

The first post came from Damian Ruff, a 43-year-old Whole Foods employee based out of Mesa, Ariz. On Jan. 23, Mr. Ruff shared an image from a family trip to Mexico, wearing a tiny sombrero and drinking a Dos Equis. His mother sent him the photo, Mr. Ruff said in an interview. It was the first time they shared a beer together after he turned 21.

“Not much has changed other than my gray hair. I see that person and go, ‘Ugh, you are such a child and have no idea,’” he said.

Mr. Ruff created the shareable story template with the picture — a feature that Instagram introduced in 2021 but expanded in December — and watched it take off.

“The amount of people that have been messaging me and adding me on Instagram out of nowhere, like people from around the world, has been crazy,” Mr. Ruff said.