Old declassified docs with blacked out portions

Occasionally the government will declassify a document either because a long time has passed or because of a Freedom of Information request. Sometimes there will still be blacked out areas because of security reasons (national security being the main reason but confidentiality of intelligence sources is frequently given).

How long after a document is declassified do they remove the blacked out portions? Are there docs from WW2 that still have blacked out portions today? I can understand about intelligence docs that go back to the 60’s because some of the people involved might still be alive or even still be in government service, but what about older items? What is the oldest declassified doc that still has blacked out portions?

I was watching something one day. I beleive it was “in the year 2000” on Conan. The line was
“In the year 2000, the government will stop highlighting things with a magic marker”

The technical word for blacked out is redacted. ISOO Directive No. 1; Appendix A Here is an article that talks about a recent incident where the CIA had some issues with redaction: http://rawstory.com/news/2007/Watchdog_says_CIA_mishandling_declassification_duties_1206.html

And see, http://findarticles.com/p/articles/mi_qa3947/is_199904/ai_n8832389

And here is the basic scheme regarding duration of classification:

Bush Executive Order 13292 on Classified National Security Information

Bush Executive Order 13292 on Classified National Security Information

*Id. *

Holy archives, Gfactor ! Thanks for the info.

Not to demerit GFactor’s excellent answer, but:
Obligatory reference to “The Onion”