Although we seldom think about them this way, most American communities as they exist today were built for the spry and mobile. We've constructed millions of multi-story, single-family homes where the master bedroom is on the second floor, where the lawn outside requires weekly upkeep, where the mailbox is a stroll away. We've designed neighborhoods where everyday errands require a driver's license. We've planned whole cities where, if you don't have a car, it's not particularly easy to walk anywhere — especially not if you move gingerly.

This reality has been a fine one for a younger country. Those multi-story, single-family homes with broad lawns were great for Baby Boomers when they had young families. And car-dependent suburbs have been fine for residents with the means and mobility to drive everywhere. But as the Baby Boomers whose preferences drove a lot of these trends continue to age, it's becoming increasingly clear that the housing and communities we've built won't work very well for the old.

A new report from Harvard's Joint Center for Housing Studies and the AARP renews this warning. What will happen to Baby Boomers stuck in suburbia once they can't drive anymore? Where will an aging population with more disabilities live when we've built so little accessible housing? How will they pay for homes late into life when there aren't enough modest-sized homes or aid programs to help afford them?

For all of these reasons, we're rapidly approaching a serious clash in America between housing stock and community design on one hand, and demographics on the other.

In 1990, less than 5 percent of U.S. counties had a population where adults over 50 made up more than 40 percent of the community (that was 156 counties). By 2010, this was true of 33 percent of all U.S. counties (or 1,031 of them):

Put another way: The U.S. population over age 65 is expected to include 73 million people by 2030 (that's about 33 million more than today). Where do most of these people currently live? In the suburbs:

And in single-family homes:

Most of those homes don't have the kind of basic "universal design" features that older residents will need, like extra-wide hallways, no-step entries, living spaces on the ground floor, or accessible light switches and door levers. The Harvard report cites 2011 American Housing Survey data concluding that just 1 percent of housing units in America have all five of these features.

In theory, such adjustments will mean making homes more accessible for everyone: for dads pushing strollers, children on crutches, anyone who just wants a grandparent to visit. But the prospect of age-proofing our housing stock and neighborhoods raises some thorny policy questions: Should communities require private homeowners or commercial builders to incorporate accessibility features? (Some already do.) Are we willing to invest in transit in communities that currently don't have much of it? Can we make room for "accessory dwelling units" — like little backyard cottages where grandma might live — that are currently illegal in many cities?

And there's another question not raised by the Harvard report, but it's an equally important one that's been voiced by researcher Arthur C. Nelson: If many Baby Boomers wind up leaving the large single-family homes they've long occupied — where they no longer want to mow the lawn or climb the stairs — will there be enough young homeowners wanting to buy them? America is the midst of shifts in both demographics and housing preferences that suggest there may not be.

If you are already worried about what an aging America will mean for entitlement programs, health care spending or the labor pool, you should probably add housing to that list, too.