They didn't actually have legal classification in the 18th century. Classification as we think of it today (where stamping something with "secret" means something lasting and legally binding and put under heavy regulation) basically doesn't exist in the USA until the 20th century. World War I was the first big spasm of classified document creation, followed by World War II (after which you have basically a "modern" looking system, with classification and declassification regulations — there were technically no standard "declassification" procedures at all until 1946).
There are definitely things from World War II that are still classified. I would be surprised if there were not a few things from World War I or the interwar period.
Follow-up question if allowed: Sometimes Classified documents are scheduled to be released on a specific date. What are the oldest documents scheduled to be released?