At age 75 in 2016, Bernie Sanders is too old to be seriously considered as the Democratic nominee for President. He would be 79 years old by the end of his term.
I really don't need to add more, but ....
There is no - none - zero example in U.S. history when a nominee that old came anywhere close to winning a general election for President.
In the 2008 campaign, one of the easiest arguments I used with undecided voters was to ask them if they knew how old John McCain was. Most said no. I then told them McCain's age (73) - and reminded the person that McCain would be even 4 years older - 77 years - by the end of his term. I then asked them if they felt comfortable with a person that old making life or death decisions about them and their families. Maybe split second decisions in the middle of the night in a national emergency. I asked them if they knew family or friends that age who they knew were "fading", no longer intellectually or physically sharp. Almost everyone had an example.
I told them of the example of President Woodrow Wilson, disabled and unable to function as President for many months of his final term.
I snagged a number of votes that way - either convincing people to vote for Obama, or if they could not bring themselves to vote for Obama, at least not take the risk of saddling the country with such a 77 year old Commander-in-Chief. [They could vote for other offices on the ballot but not vote for McCain.]
The GOP is already using her age against Hillary Clinton - and she is 6 years younger than Bernie Sanders.
I agree with 95% of the things Bernie Sanders says and does. Find someone who believes the same thing and is 15 years younger than Bernie Sanders, and you could have an argument. But Bernie Sanders, without the benefit of a time machine - a real one, not the Hollywood prop of the movie or of The Big Bang Theory TV series - does not have a realistic chance of winning a general election for President.