Age is a factor in human life. It is something thst cannot be wished away and yet one would like to remain young and youthful. Another ingradient in the life of a man is sex. It is one thing that shows how youthful a man is. Age as a factor in sex is important, but as the ancient Hindu granths say, it is the medicinal elixer that bestows youthfulness. Thus sex can never be a bar at any age. The ancient Puranas and writer Vstsyayana recommend sex with a considerable younger woman to rejuvenate a mans life.
However I will admit that most granths in ancient Hindu thought talk of youthfulness of the man and there is hardly any mention of women, except as a mean for the youthfulness of a man. A younger women having sex with a man is a panacea for remaing young. How young should a woman be in relation to a man? Generally no age is prescribed but any girl above the age of puberty can cohabit with a man,whatsever his age. VATSYANA also mentions somewhere that a 65 year old can have sex with a 15or 16year old lass. This does look abhorrent to many, but it's a biological need. Perhaps this is the unwritten law of nature that society may not recognize.