If you mean narcissistic as in self-absorbed, I don’t see millennials being that way more than any other generation. Individualism appears to encourage an “I got mine” mentality where people are overly focused on carving out their little fortress of safety and happiness with little regard for how others are doing or how their hiding within their fortress contributes to the overall state of things. But that’s more of a thing of tapping out of society, whereas, as a pathology, narcissism thrives on attention and admiration. I would think people who are going out of their way to broadcast their life choices, they’re either doing it as a vlogging thing as desperate for money, or they specifically are wanting to know how society feels about things and where they land within it. Which doesn’t necessarily have to do with pathology. I mean, remember we are social beings. And wanting approval from your peers and reassurance that you aren’t an aberration is not narcissistic on its own; I’d say it’s very normal.
In my experience with very few people who seemed distinctly pathological narcissism, it’s not just being bothered by criticism or wanting attention alone. I almost want to say it’s sort of like, if you imagine someone addicted to hard drugs, but instead they are addicted to attention and admiration. So everything in their life becomes about that, it impacts their relationships, the kind of choices they make, and they use people as sources of narcissistic supply, maneuvering them into positions where they can control them and extract from them like resources. It’s not just hurt feelings or defensiveness or posting selfies, it’s like a part of their identity and some of them become disturbingly effective at using people to feed their addiction. (Side note: Addiction may not be the best analogy, cause I’m not sure how much success attempts to heal narcissists has, but it’s the way I could think of atm to convey how deep a thing it is compared to someone who is a little egotistical or gets mad at criticism some of the time.)
Forgot to mention, people with NPD are in the same vicinity as psychopathy, with not feeling empathy or shame. That’s an important part of why it isn’t just another mental health disorder to treat, and why it’s such a serious designation, as compared to describing a person as a little vain. People who can’t feel empathy may still be able to intellectualize it, so it’s not to say they’re guaranteed to be ‘evil’ or something. And people who do feel empathy can still do horrific things. But it’s a pretty serious characteristic of the disorder.