RoadNasty
This Train'll Stop at Tucumcari
Well that's a pretty broad question. There are many levels of classification, including, as I understand, many levels above the "Top Secret" stamp. Something may get categorized in one of those levels and stay that way virtually forever, others things may eventually lose their classification over time and become public. A good example of "old secrets" becoming general knowledge is the book "Area 51", the author tapped into many former secrets from the 50's and 60's that are now public. This former sensitive material dealt with the acquisition of the lands in Nevada and the development of the U-2 and SR-71. These aircraft were ultra-critical to the security of the United States during the Cold War, but times have changed. Presently there is no need to keep a super heavy level of security on those 2 aircraft.
That being said, apparently there still exists a heavy lid on events from the 1940's involving the Roswell incident. To give you a hint, here's a short video of former Roswell deputy base commander Brig Gen. Thomas J DuBose. It was made just before he died in the early 90's, but his comments are quite insightful to the thinking that took place way back then:
View: https://www.youtube.com/watch?v=T4yVEEff8Gw