It is called Hefner's secret. Hefner, 85 year old Playboy founder established that age is not a barrier for a man's sex drive. To him age is just a number and he is getting ready for his marriage with a 24 year model. According to recent study by reasearchers at Australia's University of Sydney, testosterone level fall is due to decline in health and not due to aging. So do not be worried about your grand father's libido or consider it unusual. Just be comfortable to know he is healthy and hale.