I look more at how often a candidate does a stark 180 degree turn on their rhetoric, vis-a-vis their prior political track record.
Just to pick on Mrs Clinton a little-- She went on national television and asserted that a marriage is only between a man and a woman, but then later recanted, despite years of politics that suppressed gay people-- because supporting gay marriage became politically fashionable.
The less a candidate does that, the more I consider them to have integrity, and to have genuine views on a given subject matter.
Sanders has warts, sure. Every politician does. However, his prior voting history and session minutes history reveal he has always been stalwart in seeking protections for common Americans, and warned against the egregious actions of private interests meddling in politics.
Given the current political climate of powerful multinational corporations basically dictating domestic policy, and an ever eroding set of civil protections from said multinationals and their policies, I would say we need somebody like Sanders more than ever.
However, without the many years of service in the legislature, you have no means of qualifying the candidate's actual position or platform, which means if you are serious about your pick, and not voting with rose colored goggles on, you end up voting for old people that may be less adaptable and capable in changing or turbulent circumstances (and may well come with a raft of age related illnesses that could complicate their leadership capacities.)
Short of everyone developing true telepathy, I dont see a solution to this problem.
It's why I am willing to overlook Sanders' age. I would rather have his strident resolve for restoration of social protections and his commitment to restoring oversight against large actors than have somebody young and attractive.