The idea of older guys having (or, rather, not having) sex used to be a popular source of humor. It's probably because, in the days before Viagra, so few men maintained sexual vitality into old age. Even when I was growing up, jokes about an older guy unable to keep up (or get it up) with a young wife were pervasive.
Sex scientists and reproductive medicine researchers have since made this kind of humor obsolete by inventing new drugs and treatments that aid men in retaining virility throughout life.
Of course, medicine isn't the only answer to maintaining your virility. Regular masturbation and overcoming sexual guilt and shame transforms lives.