A recent question (https://www.reddit.com/r/AskHistorians/comments/6aanfn/are_there_still_any_top_secret_or_classified/ ) asked if any files remain classified from the WWII era, to which the answer is (in brief), "yes".
Do we know what the oldest files are that remain classified and inaccessible to those outside the US government without the proper security clearance?
This has been asked a few times; here is one of the more recent answers (which I answered). The basic answer is, in the US, legal classification of documents mostly dates from the early 20th century and the stuff which is longest-lived is of a technical nature, e.g., signals intelligence/cryptography/gas warfare. So there may be some World War I things still classified, or interwar period stuff. There is certainly a wealth of stuff from World War II.