Two seniors in their 70's one widowed one divorced love each other. They decide to move in together. They have no desire to get married this late in life. They have separate bedrooms, no sexual relations, no joint bank accounts but truly love each other and for the most part are viewed as in a relationship.
Are they living in sin?
I do not know anyone in this situation but I'm fairly certain some are. My underlying question what exactly is living in sin? Is it a sexual relationship that makes the difference? Would we look at it differently if the two were 30 years old?
The answer is no. It does not matter if they are 30 or 70.
Hypothetical but keep in mind that the Bible, Old or New Testament does not have a requirement for weddings or vows. That requirement is made by the Protestants denomination in the 16th century so that means a lot of marriages without weddings. If they do have sex they are married. So there is no sin in this scenario. Someone to be with and love and be loved at 70, I would say that is a blessing.
But just like everyone else the issue is if they separate. The discussion in the Gospels where Christ, a Jew, is talking to Jews, about the Mosaic Law concerning divorce, does not apply to Christians and the word divorce does not appear outside the Gospels. Some Bible translations put it there, but it is not in the actual scriptures. Christianity did not use the "Letter of Divorcement" and there is no courts to litigate those that separated.
Now Paul does address some of this in 1st Corinthians chapter 7, along with His view of virginity and circumcision and the end of time close at hand..."time has been shorted." And then He says that the unbelieving spouse is sanctified by the believing spouse, which is really something. But still does not mention the word divorce, instead the word release or loose is used. And of course recommends reconciliation. But then along with Romans 7:1-3 that says a wife cannot remarry. Either way married people are suppose to stay married. But to the married I give instructions, not I, but the Lord, that the wife is not to leave her husband 11 (but if she does leave, she must remain unmarried, or else be reconciled to her husband), and that the husband is not to leave his wife.
This opens up another discussion, but for the fundamentalists and others, this is it.