Indeed, there are many great and younger candidates out there. The main issue for the 2016 elections, however, is that almost none of them has universal and national name recognition. This counts for a lot, as a little-known younger candidate who was courageous enough to throw in her hat at a very late date for the MT US Senate race can tell you well. She was literally Montana's Elizabeth Warren and worked tirelessly for the two-month period that she had to make herself known.
Whenever I tried to talk up her candidacy here on DU, I was mostly slapped down by others who insisted that their money would be spent better elsewhere. We saw how well THAT went in November. The national organizations also did zilch to help her. The only outside the state supporters were DFA and Emily's List and even DFA never listed her as an endorsee, although DFA certainly helped with initial fund-raising. To me, a Dem Senator is a Dem Senator and has equal weight whether that Senator is from Kentucky or from Montana. Amanda Curtis was an exceptional candidate. The odds were against her this time, but I'm sure that we'll see her again. We need people like her.
I was born even earlier in the 40s than you, "young'un"! 
We should not count any of our candidates before they have officially thrown their hats into the ring. But, in answer to your question about why Hillary's declaration is almost universally disbelieved while Elizabeth's is not, I believe that the reason is twofold: a) Hillary is literally available - no Dem Senate, US Rep or Governor seat would be in jeopardy if she runs; and b) Hillary has actually run for President already.
In any event, if the GOP manage to steal the 2016 election, we will ALL be left to gnash our teeth and that's in best case scenario. ANY Dem candidate would be better than any GOPer. And that's the truth. We Dems need to keep our eyes on the BIG picture and not engage in petty bickering about which candidate among the many qualified Dems we should support. We should support the Dem nominee. Period.
I am not saying that I believe that you are engaging in petty bickering. Not at all. You raise excellent points. But too many here are. That really has to stop. We can certainly support whichever declared candidate we wish as heartily as we wish through the primaries. But even then, we should stress their positive aspects and why we support those particular candidates and not raise negative aspects for the others. The GOP already does enough of that - why help them do their worst?
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