I have seen entry floors so rotten that they flex and sag greatly when walked on. Even on houses that are only a few years old. There is a slight bit of bounce and flex allowed in floors, but if you have noticeable movement in your floors, you have a problem. Bathroom and entry door floors are probably the worst areas for rot problems. Any areas where excessive moisture can get to the floor.
Wet crawlspaces with inadequite venting and moisture management, entry doors flashed wrong, and poor deck installation all can lead to this problem. Some doors and decks look fine from a distance, but if flashed wrong, can almost funnel water under the door and behind the siding, wrecking your floors.
Of course bathrooms can be tough for moisture management because of the water used in them. Mold or mushrooms growing anywhere in your home are always a bad sign.
I am not trying to diagnose every rotten floor situation here, just know that if you have a floor that you notice sag when you walk across it, you have a problem and should get it looked at.