The HNN article notes:
The dwindling ranks of our World War I veterans – literally from millions to one – marks a poignant moment in our nation’s history. When Frank Buckles is gone, our direct and living connection to the Great War will be gone. Only images, artifacts, and words of the period will remain.
When Frank W. Buckles dies, his name will be added to a list of "last veterans" kept by the U.S. Department of Veterans Affairs. The most recent update to this list took place in 1992 when Nathan Cook died at the age of 106. He was the last surviving veteran of the Spanish-American War.
(Frank Buckles at age 103. Quite the dapper gentleman!)