It's not like an ordinary job where the people who hired you can observe you daily and fire you if you don't seem to performing up to standard for any reason. Once the President is elected, the White House protects him and impeachment is the only way to remove him or her, absent resignation.
The health of President doesn't usually make news except for the first time he runs and if there is a problem that we learn about. In the meantime, the public is in the dark and isn't even aware of it because its attention is not focused on the health of the President on a daily basis. Even if it wondered about health daily, how would it find out?
Recently, I've been watching West Wing, where the President did not affirmatively disclose his multiple sclerosis. Because it was dormant most of the time, it would not show up on his annual physicals, which showed him to be in excellent health, which the press secretary would announce.
In real life, Paul Tsongas ran for President. He disclosed that he had had cancer, but apparently did not give all the details. I once met the woman who had his medical files locked in her desk. He ran for the election and died in 1997. I don't know at which point he became incapacitated. Had he won the nomination, presumably the party and country would have expected him to serve a full term at peak health and to be able to serve a second term, if re-elected. And they would have been SOL on that account.
http://en.wikipedia.org/wiki/Paul_Tsongas
More importantly, we have reasons to believe that Reagan's Alzheimer's was affecting him while he was in office, but concealed by his wife and staff. If so, people for whom not a single American had voted may have been having the final say on decisions.
On the other hand, McCain, whom many Democrats considered too old to run in 2008 seems as healthy as ever. So, we don't know.
So, we have to look eight years out from election day and play the odds and pray or cross our fingers, or whatever our beliefs lead us to do.
This go round, all the candidates but O'Malley are older than I personally consider optimal, including my own favorite candidate. So is Warren, who did not announce, but whom many wanted to run. It is what it is. But that doesn't mean we kid ourselves.