Does the rating scale change in time? I mean if X stars from for example 2000 are less safe car than X stars from 2013? I am mostly interested in adult safety within frontal (and side) impact and the ability to compare these ratings between various dates.
Yes, they do change over time. Initially NCAP was more focused on the safety on people inside the car, but these days there's also a focus on the safety of people you may drive into. I can't recall the exact year, but it was around the time cars' noses seemed to be more slanted and to dip lower that the NCAP test also started focusing on cars having to NOT run over pedestrians if they knock them, but rather scoop them up and deposit them on the bonnet. If your car scored a 3.5 the previous year, but it didn't perform well in the pedestrian test, you'd probably only get 3 stars this time around.
IN light of this, there may also be other things that change, either becoming MORE or LESS important. Like ABS and EBD for instance.