According to this executive order:
Records exempted from automatic declassification under this paragraph shall be automatically declassified on December 31 of a year that is no more than 75 years from the date of origin unless an agency head, within 5 years of that date, proposes to exempt specific information from declassification at 75 years and the proposal is formally approved by the Panel.
So, there are cases when the information might not be declassified even after 75 years (if I understand correctly - the paragraph is not clear for me).
However, according to this archive article:
The Order also recognizes that some information might remain sensitive and pose a threat to the national security if released at the 25-year mark such as information which reveals the identity of a confidential human source, human intelligence source, or key design concepts of weapons of mass destruction, which may have a duration up to 75 years.
Q1: Is 75 years a maximum threshold for information declassification or it can be delayed indefinitely?
Q2: How was this 75 year limit chosen? For me it seems about the life expectancy in developed countries.