I would like to see younger candidates but for slightly different reasons
To state the obvious, we are not hiring someone at the age they are at the beginning of the term. We are hiring them for at least 4 years. I want a candidate with the best opportunity to serve effectively for 8 years. That means alive, mentally sharp and not suffering some debilitating disease. It is simple fact that someone has a better chance of doing that during the 50-65 range than the 70-85. I am not worried about Bernie being 79 when he starts. I am worried about him being 87 when he finished. I want to back a candidate for the long haul.
Second, I think that a good candidate has a superstar quality about them that generates high energy and enthusiasm in supporters (ie the IT factor). Forty years of increasing levels of responsibility in government will build a solid resume but no amount of experience will get you the IT factor. The thing about those superstars is that they get tapped early to rise to the top. Obama could have spent 20 years in the Senate and run for President as a ultra-wise 65 year old. My point is that people with his level of political appeal don't stick around that long.
I am open to any Democrat and will support whoever gets nominated. But my preference is for an up-and-comer to come out of nowhere and really grab the party.