@Blickers,
Using that example, someone would have had to break through at the last convention and what did the Democrats present? more oldies
No one young and wildly exciting at that last convention.
The Democrats haven't been doing their succession planning.
Now the Republicans went outside of their succession plan - and picked someone who's been aiming for the job since the 1980's - but they do have some younger candidates and potential candidates lining up behind #45.
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The American system does make it more difficult than other first world countries for young candidates to appear but a few have made it.
My preference is for national leaders in the 35 - 50 range so they're done before they're 60. That means the Democrats need to be developing potential candidates who are now 28 - 40.
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edit: so to the original question. The Democrats don't appear to have any good candidates in the pipeline. The Republicans have a sitting president as well as a big batch of contenders (some of them were in the last round and hopefully learned something about debating/campaigning). There's always Pence, but he's truly dreadful. He's got potential to do serious damage.