The floors from many houses built in Sixties are made from fairly thin chipboard and so are liable to creak very badly after time. Modern chipboards are at least the standard 18mm thick, though these are still not recommended for domestic flooring if they are in tongue-and-groove format.
Chipboard itself is known to a phenomenon horribly called ‘creep’ (!), wherein the chipboard tends to sag under its own weight as the years go by.
The only foolproof solution is to replace the chipboard with proper pine boards as a sub-base.