For most of the last seven decades, multi-generational households in America likely contained an elderly parent or grandparent, an older generation cared for by the younger one. But in just the last few years, a curious trend has emerged: Young adults, aged 25-34, became more likely to live in a multi-generational household than the elderly over 85.

In reality, this means that a grandmother over 85 is more likely to live away from her family than an adult child of 25. Just barely. Here is how the Pew Research Center charts this odd reversal in a new report on multi-generational living in the U.S.:

Those two trends, drawing on Census data, reflect some long-running and fundamental shifts in the economic prospects of the old and young. But before we get to them, a couple of quick definitions. Pew's "multi-generational" household does not refer to nuclear families of parents with young children living at home (although, yes, that would be two generations). Pew also takes a more expansive view of what "multi-generational" means than the Census Bureau does.

Here, it's counting several different kinds of family arrangements: 1) parents living with their adult children over age 25 (a scenario in which either adult or child may be the head of household), 2) households with three generations, including parents, adult children and grandchildren, 3) "skipped" generations such as a grandparent caring for a grandchild, or 4) households with more than three generations. College students living in dorms, or young adults 18-24 still living at home, are not counted in the analysis.

So what's happened here? On the one hand, fewer and fewer of the elderly have been living with family for decades now, for a variety of reasons. Advances in medicine and health mean that it's more likely now that an adult over 85 can live independently. Social Security and pensions have given them the financial stability to do so. And as the average size of the U.S. family has declined, elderly parents now also have fewer adult children with whom they might live.

Meanwhile, the likelihood that a young adult will live in a multi-generational household has been increasing since 1980. This has been particularly true over the course of the recession, as job prospects have dwindled and as twentysomethings have been increasingly delaying the markers of adulthood (first job, first house, first kid). But the economic prospects of young adults have been shifting for much longer. As Pew points out, poverty has fallen over the last 50 years for the over-65; but it's been rising since 1980 for young adults.

Here's the troubling bottom line, from Pew:

While the likelihood of residing in a multi-generational household may not be a direct measure of economic well-being (or lack thereof), there is evidence that the changing patterns of multi- generational living parallel “the general trends toward the greater economic security of older adults and the increasing financial strain experienced by younger adults.” (Kahn, et. al., 2013)

This trend nests within some much larger shifts in multi-generational living. Nationwide, the share of multi-generational households has been on the rise since 1980. And it will likely continue to increase, even as the economy improves, because immigrant and minority households that make up a growing share of the population are more likely to live in multi-generational households. This has some interesting implications for housing itself: Demographics can shift faster than our housing stock does. And so how will communities full of tw0-and-three-bedroom single-family homes accommodate families that want to pile in more than one generation at a time?

The most unnerving takeaway, though: Changing living patterns may be another sign that a country (and economy) that has evolved over the years to support the elderly doesn't do quite such a good job with the young.