What are his chances to win Democratic nomination ?
As alluded to above, bad, in large part due to his age. He could be a brokered convention compromise candidate with a young, progressive (or progressive-ish) VP.
A more credentialed Joakim Castro. Beto O'Rourke if he pulls a miracle over Cruz. Kirsten Gillibrand. Jason Kander if he had won his Senate race.
I've run the largest state in the country for over a decade cumulatively. Experience, experience, experience.
Poor. Voters seem not to really care about experience and tend to prefer charisma. Brown has the former in spades, but relatively little of the latter.
Experience
Age.