Granted I'm still in college so I'm sure the other Dems I'm around are younger, but despite Bernie's age he still has massive support. I think it could come down to Bernie/Biden in 2020.
Regardless, someone will need to step in and carry the torch of the left flank of the party. Think we need to see a consensus in party of single payer healthcare, tuition free college (for the record I'll be finished before this would go into effect), and massive tax reform to reverse current/Reagan Era changes.
Also, another vibe I get is that many of my younger Republican friends are absolutely done with Trump. Now, that doesn't mean they will vote Dem. But these are people who voted for him in 16 so that's good news at least.
Many Older Republicans seemingly will always support Trump but definitely enough instability for a strong Dem to win easily in 20.