Atleast twice a week
Really, there's no rule for it :) I'd say that as often as they can and they feel the need.
Not sure if there is such thing as "too old for sex": http://forums.imyours.co.uk/showthread.php?2130-How-old-is-too-old-for-sex
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them