1975? Ok, most unlikely to be floorboards then. Probably chipboard. Chipboard loses most of it's strength if subjected to water for any length of time.
If you can imagine the floor joists set at roughly 400mm apart, then the thing to look for is any dipping (under your weight) between each joist. A sure sign of weakened chipboard.
The only way around that is to either replace it, or put another layer on top.... but that would raise the floor level by 19mm.
If not, and the whole lot is bouncing, then the solution is pretty simple..... take up the covering .... cut a few trapdoors in the centre area of the room ...... then reach into the floor void to insert a few simple vertical props (pieces of 4 x 2). (Assuming this is on the ground floor)
Worst case is substantial rot in the floor joists. Unlikely in a 40 year old house... depends on whether the vents have been kept clear over the years.
Nothing to be decided until someone sensible has a proper look... probably an "old school" local builder, or anyone with some experience.