Other researchers who had been exploring the link between cohabitation and divorce failed to take into account the age at which couples took that plunge. Kuperberg wondered if once she controlled for age, the link between cohabitation and divorce might disappear.

Using data from the U.S. governments’ 1995, 2002, and 2006 National Surveys of Family and Growth, Kuperberg analyzed more than 7,000 individuals who had been married. Some of the people she studied were still with their spouse. Others were divorced. Then, instead of studying just the correlation between cohabitation and divorce, Kuperberg looked at how old each individual was when he or she made his or her first major commitment to a partner—whether that step was marriage or cohabitation.

Moving in together without a diamond ring involved didn’t, on its own, lead to divorce. Instead, she found that the longer couples waited to make that first serious commitment, the better their chances for marital success.

So how old should couples be when they commit? The research shows that at 23—the age when many people graduate from college, settle into adult life and begin becoming financially independent—the correlation with divorce dramatically drops off.

Kuperberg found that individuals who committed to cohabitation or marriage at the age of 18 saw a 60 percent rate of divorce. Whereas individuals who waited until 23 to commit saw a divorce rate that hovered more around 30 percent.

“For so long, the link between cohabitation and divorce was one of these great mysteries in research,” Kuperberg says. “What I found was that it was the age you settled down with someone, not whether you had a marriage license, that was the biggest indicator of a relationship's future success.”

Cohabitation has become so common that it’s almost odd not to test drive a partner before marriage. It’s worthy of a People magazine headline now when a celebrity couple “waits until marriage” to shack up. Bachelor Sean Lowe (of ABC’s The Bachelor) and his wife Catherine Giudici were all over the tabloids when they announced they would not move in together until after their televised wedding.

Cohabitation has increased by nearly 900 percent over the last 50 years. More and more, couples are testing the waters before diving into marriage. Census data from 2012 shows that 7.8 million couples are living together without walking down the aisle, compared to 2.9 million in 1996. And two-thirds of couples married in 2012 shared a home together for more than two years before they ever waltzed down an aisle.

Today, discussing cohabitation is about as salacious as watching grass grow. A 2007 USA Today/Gallup poll found that just 27 percent of Americans disapproved of it. The number of painful discussions I personally endured two years ago when I moved in with my own boyfriend can be counted on one hand. My fridge is littered with wedding announcements from couples who are engaged and lived together for years.