“It used to be called shacking up, and it was not approved of,” said Kelly Raley, a sociologist at the University of Texas, Austin, and former editor of The Journal of Marriage and Family. Families and religious groups often condemned living together outside marriage.

But Americans are far more accepting now, she said, and the people turning 60 “are very different from the people who were 60 twenty years ago.”

Karen Kanter, for instance, had divorced twice after long marriages — 38 years, in total — when she met Mr. Tobin on Match.com. “Getting divorced gives you so much to untangle,” she said.

“Our life is good together, so why disturb it? I just don’t see the importance of that piece of paper.”

Mr. Tobin, also divorced after a long marriage, wouldn’t mind marrying his partner — he actually proposed on bended knee once, though he knew Ms. Kanter would say no — but he is also fine with cohabiting.

“The relationship is looser,” he said. “We don’t make demands on each other’s time. She has her life, I have my life, and we have our life together.”

For older people, the advantages and drawbacks can stack up differently than at earlier ages, when such relationships tend to be more unstable. Demographers see most youthful cohabitation as a prelude to marriage or simply a short-term arrangement.