All those of whatever age and circumstance are called to live chastely. For those who are not married, that means no sex. Being elderly doesn’t give one a license to sin. The young folks call what you’re describing as being “friends with benefits” and God doesn’t think that’s funny.
Where did you get the idea that fornication is OK just because you’re old? If you’re both widowed you are free to marry even though it may be complicated by family and financial considerations.
I edited this because I reread it and it sounds harsh. That’s not intentional. Being a faithful Christian isn’t easy. Being left alone after a spouse dies is part of the cross one carries and it’s not a light one. May God bless you both.