The probability of either pilot becoming physically incapacitated is extremely low, even if both pilots are over sixty. Most accidents occur due to pilot error rather than pilot incapacitation. There is a strong inverse correlation between pilot experience and pilot error. Pilots over 60 are usually slightly more experienced than pilots under 60. And in commercial aviation, experience and competence with procedures and systems is more important than physical condition (beyond a general state of reasonable health).
Given all this, a cockpit with two pilots over 60 may actually be safer than one in which one or both pilots are under 60, because it is extraordinarily unlikely that they will be incapacitated, and their combined experience will have more of a bearing on any unusual situations than their state of health.
Health requirements for pilots are too strict in some ways, and not strict enough in others. A person who had a heart problem briefly twenty years earlier may be disqualified, but someone who smokes five packs a day or drinks himself into a stupor on his days off may still be able to get a medical. The regulatory requirements don't always make objective sense, and, as alluded to above, health problems are way, way down on the list of causes of accidents.