I've heard sexual desire is the last to go, even after old age releases the social inhibition on saying or doing rude things like groping the nurse at the old-folks' home. So it is obviously natural (except probably not desirable to grope unwilling women, of course). As far as what frequency, if someone's ability is still present and he has a willing partner, as often as he can find someone mutually willing to engage in the activity.
Whatever feels right for you and your partner is your answer.
Yes, a perfect answer from NCB22.