1418
"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".
I don't consider different eras as different nations though. I think that's splitting too many hairs. I see a nation as a country that is generally united and governed by a leading entity.
Going back to the Japan example, I would consider them a nation when all the clans were united under one rule. Same with UK, India, Thailand etc.