It's an interesting dilemma. In the airline industry, commerical airline pilots are REQUIRED to retire at age 60 by the FAA. It doesn't matter how fit or experienced they are -- there are no exceptions. In fact you'll remember two incidents some years ago - a United B-747 out of Hawaii that had a cargo door blow off; and a United DC-10 that lost all hydraulics over Iowa. In each case, the Captain saved hundreds of lives through ingenuity, experience, and shear guts. And each was then forced to retire not long after thanks to the Age 60 Rule.
The FAA says that after age 60, pilots' physical ability, on average, declines. So they have drawn a line in the sand which no pilot may cross. And the courts have refused to overrule the FAA, despite the fact that there is tremendous scientific evidence that pilots can perform superbly after age 60.
It's interesting that a majority of pilots SUPPORT this rule, since there are more first and second officers than captains, who gain by captains' retirements at age 60! If the age limit were raised, there would be fewer promotions.
So where there is always more than one crewmember in the cockpit in a jetliner -- just in case one has a heart attack or other physical problem, there is an age cut-off anyway. But in the cab of subway train, if you have a heart attack, just hope that the deadman's feature works... and that the tripper system/brakes work well enough to stop the train (unlike on the Willy-B!).