1413
"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".
If changing territory resets the count, then the US is only as old as when Hawaii or Alaska joined (I think Hawaii was the last addition? Dunno, I'm not an American...)
They were both in 1959, but Hawaii was later in the year.