6. If the husband or wife is not fertile, due to injury, illness, or old age, is the act of natural marital relations still moral?

Yes, natural intercourse is still moral, even if the husband or wife is not fertile due to injury, illness, or old age.

Pope Paul VI: "The sexual activity, in which husband and wife are intimately and chastely united with one another, through which human life is transmitted, is, as the recent Council recalled, 'noble and worthy.' It does not, moreover, cease to be legitimate even when, for reasons independent of their will, it is foreseen to be infertile. For its natural adaptation to the expression and strengthening of the union of husband and wife is not thereby suppressed. The fact is, as experience shows, that new life is not the result of each and every act of sexual intercourse. God has wisely ordered laws of nature and the incidence of fertility in such a way that successive births are already naturally spaced through the inherent operation of these laws." (Humanae Vitae, n. 11.)

In order to be moral, each and every sexual act must be marital and unitive and procreative. This is the threefold object of every moral sexual act. This natural sexual act is procreative precisely because it is inherently directed toward procreation. In other words, it is the type of act that is intrinsically ordered toward the good end of creating new life. But even when this act does not or cannot achieve this good end (its moral object), the act remains inherently ordered toward that same end, and so it retains that good, the procreative meaning, in its moral object. An act does not have to achieve its moral object to be inherently ordered toward its moral object.