Shalom, Zera Le'vatala (waisting seed not for the sake of procreating) is valid for everyone, does not matter how old they are. If an older couple are having marital relations for the sake of the Mitzva that is called 'Ona' (עונה) - meaning, pleasuring the women with marital relations (which is a Mitzva and the man must provide his wife with marital relations) as long as it's done the right way (and one must know all the laws what is considered 'the right way') then this is not considered Zera Le'vatala. But if the couple (that can not have children anymore) have marital relations and the sperm is not placed in the women, this will be Zera Le'vatala and it has nothing to do if the women can or can not get pregnant In all cases married couples must know all the details how not to fail in Zera Le'vatala All the best, R. Anava