My house was built 3 years ago and the second floor squeaks and gives when walked on. I am not talking about one area but many many areas.
Floor joists if properly sized and spaced should not noticeable deflect when walked on and squeaking could result. Squeaking could also occur from movement of floor boards across the joists, if not nailed securely or if the spacing between joists are too great. Normally a bedroom floor would be designed for a live load of about 30 lb./s.f. The size of joists and their spacing would depend on their unsupported span.
