The theory of generations suggests that each successive "generation" experiences a dialectical reaction to the generation and cultural norms that held the most influence in society when they were young, on the one hand absorbing some of their values and on the other reacting against their perceived excesses. I think that's the only sense the term "millennial" is useful, it isn't just people born in a certain year, its people whose dominant influences were the same and came from the same place. It also doesn't remotely represent the majority of people born in a certain year, it's extremely weighted towards the middle class for example (because the middle class tend to be culturally hegemonic). It also tends to revolve around youth stereoptypes and doesn't reflect how generations evolve as they age, generation X is still defined by 80s and 90s social stereotypes about punk, grunge and yuppie culture, but many Xers are in their 50s now and there's little sociological interest in how the social stereotype has changed during that time.
In short, treat the whole thing as an interesting ideal type which can teach us a bit about shared formative experiences. For example, the stereotype about millennials struggling financially and not having housing is clearly a reflection of the fact that millennial were entering the job market or early in their careers when the 2007 financial crisis happened. The stereotype about all millennials working in media or creative professions reflects the fact that millennial were the first young adults to enter the workforce with some exposure to the internet and new media, and the stereotype about millennials being particularly concerned with social issues comes from the fact that millennials directly experienced the "turning point" of perception of those social issues.
But I think it's important to remember that milennials will (and already are) going through the same transformation as every previous generation. They will stop being young outsiders reacting against someone else's cultural and economic dominance, and they will become the culturally and economically dominant force in society, and I worry that when that happens a lot of the idealism of milennials will turn out to be as hollow as that of previous generations.