804
"No nation is older than 250"
(lemm.ee)
Transcript
A threads post saying "There has never been another nation ever that has existed much beyond 250 years. Not a single one. America's 250th year is 2025. The next 4 years are gonna be pretty interesting considering everything that's already been said." It has a reply saying "My local pub is older than your country".
China gets a bit fuzzier in between dynasties and revolutions. But there are definitely multiple post-Unification dynasties that lasted longer than 250 years.
What about San Marino?
Absolutely.