Given two people (one of each sex) who are both well past the age of conception, does their fornication (they are not married to anyone, both being widowed) constitute a sin, mortal or venial?
The act of sexual intercourse is the physical expression of vows of marital commitment made at the altar. It is a holy thing. Such behavior outside of the context of those vows is a lie in that it represents nothing more the desire for the pleasure of the moment with another to whom one is not married. It is a mortal sin–regardless of how old the individuals may be.
Yes fornication, of itself, is always a mortal sin.
Fr. Vincent Serpa, O.P.