People stop having sex when they stop breathing. There is no age limit - remember sex isn't just about full intercourse, so even if the male isn't "up to it" any more it doesn't stop the couple having a full sex life. There is no average age for people becoming disinterested in sex as it is down to the individual and external factors should be considered (as previously mentioned) menopause, post natal depression, lack of sleep, stress, middle-age spread can all be factors in losing one's libido. And finally we stopped having kids 18 months ago and my wife can't keep her hands off me.....