Popular? Just look at all the states he's won.
But seriously (a little), Trump is the most popular, and not exactly young. That youth thing was my own opinion of what the GOP needs, not an endorsement of any current candidate. I think Mitch Daniels is more connected with a youthful perspective, although not objectively young.
I think any discussion of Bernie's chances is academic....if it's not, I reserve judgement. BUT, I think Bernie does well in NY, FL, OH and CA against almost any republican nominee. Clinton will have trouble with OH and FL and perhaps CA.