Yes, a perfect answer from NCB22.
Whatever feels right for you and your partner is your answer.
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.