Demographers like to use 20 year periods for defining generations, but it has been a little messy over the years. For instance, Baby Boomers are the constant, with generations before them and since being measured from them. For years, the Boomer generation was recognized as those born from 1946-1965, but in recent years, some have started using 1964 as the end of the generation, shortening it to just 19 years. There are some as well that maintain the Boomers actually began before the official end of the War and date it from 1943.
The point is that defining generations is tricky, and so many people place great credence in what a generation is - applying those labels to social and consumer behavior. Homes, car and trucks, music, TV, technology, insurance, and so many other product preferences are defined in terms of and aimed at (in terms of design, color, pricing, features, and advertising) various market segments otherwise known by their generational name.
Baby Boomers have been the standard for decades. They have been, until recently, the largest generation so it is no surprise that retailers, marketers, advertisers, and product designers have focused on the buying power and needs of the Boomer generation. Initially this was aimed at their parents, then the teenage Boomers, the parent Boomers with children of the own, the homeowner Boomers, the prime earning Boomers (interested in travel, investments, entertainment, and property), the early retiring Boomers, the grandparent Boomers, and the senior Boomers (with AARP, health care, second careers, and more).
Because each generation is 20 years or so in length, the Boomers, like those before it and the Gen Xers (although they got shortchanged into as few as 16 years) and Millennials since, have those at the beginning of the generation in age and those at end, as well as people spread throughout. By the time, the oldest Boomers were graduating college, beginning careers, and getting married (some, not all), the youngest were just starting or getting ready to begin grade school.
This is why we shouldn't get too excited about the Millennial market. There is a concern that they are putting off buying homes and that they are redefining other types of purchases. There is an observation, and measured observance as well, that a large portion of them are still living at home or have returned there after finishing college, the military, or a broken relationship.
Maybe the Boomer generation (the ones doing the observing and the analysis) are remembering (accurately or not) they way they behaved at the same point in their lives and are finding the Millennials very different and not measuring up to expected behavior in several ways. This may be a true generational difference, or it may be a temporary adjustment the younger generation is going through ("a phase"), or it could be an over-generalization based on the data or observations used.
Regardless, remember that the oldest Millennials are in their mid-thirties in age while the youngest of them are still in high school. No one would expect highschoolers to be purchasing a home or living anywhere but with their parents. Perhaps there are some differences in the way the 30-something Millennials act and their worldview and the way it was with their parents or other Boomers when they were that age (or at least the way they remember it). It could be a true generational shift, a blip, or just they way it is being observed.
One thing is clear. Many of the Millennials are simply too young still to be doing anything but finishing high school or going to college. We have social media, instant-access news, polls, surveys, focus groups, and other ways of observing behavior today that either didn't exist or weren't used to this extent some 40 years ago when the oldest Boomers were in their mid-thirties or even 20 years ago for the youngest ones in this group. It's entirely possible that we just know more about the Millennials and their behavior in an observed, real-time sort of way that we didn't have the ability to do at the same point in the lives of the Boomers and those from previous generations.