They don't have a deep bench of candidates or leaders who are younger. The base may be younger but diverse and fragmentated with no one clear leader. Frankly in the leadership department they have a staffing issue.
I think the truth of the matter is Clinton ran unopposed not because no one wanted to challenge her, but perhaps there was no one to challenge her ... except for crazy Bernie. They tried to get Warren but she wasn't interested. Had Uncle Joe's son not died would he have run?? I'm thinking probably not. I'm sure right now they are scrambling to see who they can run in 2020; one of the Castro brothers perhaps or maybe Kaine. Clinton was their ace in the hole, no one, especially the DEMS thought she would lose.
IMHO Bammy was so bad that even the DEMS have turned away from the party.