Now is that nice?
What if I where to say, "I guess it's really true what they say about the stupidity of the young and inexperienced people, they don't listen to all sides of the story, only the side that works for them."
But I would never accuse anyone of being stupid because they are young, any more than I would them being stubborn because they are old because it's just rude, and actually, in itself a cop out excuse to dismiss what they claim, as in, while in an argument, if I were to say:
"All young people are ignorant because they are young" as means of discounting a perfectly good point they may have.
See how that works?