"Older consumers are carrying more mortgage debt into their retirement years than in previous decades," the Consumer Financial Protection Bureau said in a recent report. The percentage of homeowners age 65 and older with a mortgage rose by 8 percentage points in a decade. As of 2011, fully 30 percent still carried mortgage debt, as did more than 21 percent of homeowners 75 and older.
"During the last decade we went through this shift," Harrell said. "Before that, most of the homeowners 50-plus owned their homes free and clear."
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The percentage of older Americans who live with non-romantic housemates has stayed about the same, hovering around roughly 1 percent of the population 50 years old or older, Harrell said. Today, though, the absolute number of people living in these circumstances is greater because the number of older Americans has increased as baby boomers have aged.
Staci Gallardo, a landlord in Northern California, rents rooms in an investment property across the street from her own home to senior citizens who can't afford to go anywhere else.
"Most are on disability [or] they have health issues," she said. "They need a place to go. They have a fixed income and apartments are kind of pricey," she said.
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Most are entirely dependent on Social Security or disability for income, although Gallardo said one of her tenants helped her out with yard work to earn a little extra money. If her tenants had to pay rent on their own, "I don't see how they could have an income of between $900 and $1,100 a month … and have money to eat and pay utilities."
"Absent adequate savings, many are compelled to return to work [or] are renting out rooms … to make do," said Robert B. Goldberg, senior director of legislative affairs for the Jewish Federations of North America, which has been involved in promoting programs and policies to help seniors stay in their homes.
Rita Miller, 84, shares her rented apartment in New Jersey out of financial necessity. "I do have Social Security and some money from the sale of my house, which I am living on and is going too fast," she said in a conversation that took place over email, since Miller is deaf.
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This subletting arrangement covers about one-third of her rent, Miller said. "If I did not have the tenant I would not have enough money to stay here," she said. "I really have no objection to assisted living if I came to need it, but a lot of the places I have investigated are out of my financial range so I do not know what [I] would do."
Aside from the extra measure of financial stability her tenant gives her, Miller said she's grateful for the sense of security, as well. "I am really quite independent so I do not depend on my tenant, but it is just nice for me to know someone is here," she said. "So far I have not had any emergencies that required her to help me, but it gives me peace of mind to know someone is here."
In addition to financial reasons, this peace of mind is another key benefit for seniors worried about living alone.
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"It does allow her to stay in her home instead of going into a facility," said Gavin Grooms, a 50-year-old Provo, Utah, resident who advertised on Craigslist looking for someone to share his 85-year-old mother's house. Although Grooms lives nearby, "A big part, for me, is making sure if there was an emergency, that she's got someone who's right there who could get ahold of me quickly," he said.
Grooms' mother has COPD, no longer drives and relies on oxygen, but Grooms said she still enjoys hobbies like gardening and values her independence. "She doesn't want to feel like she's being nursed at this point in her life. She's not ready for that."
—By Martha C. White, NBC News