What I am trying to understand is the following paragraphs:
Previous studies compared the divorced rates of couples who cohabited with those who didn’t by using the age of marriage. Kuperberg did something new: She compared the relationships using the date of first moving in together. That date, she reasoned, is when a couple really takes on the roles of marriage, regardless of whether they have a legal certificate.
Using this method, she found no link between whether people had cohabited before marriage and their rate of divorce. The turning point in age for picking a life partner seems to be about 23, Kuperberg said.