The front runners in the primary are Katie Porter (49), Adam Schiff (63), and Barbara Lee (77). All three are currently in the house of representatives.
So long as people retire when they start to go downhill mentally, peoples positions are much more important than their age. Most progressives would much rather have President Bernie (82) than President Tulsi Gabbard (42).
Lee has been pretty consistently progressive. She was the only vote against the authorization of force after the September 11th attacks, she was a founding member of the congressional LGBTQ+ equality caucus, and volunteered with the Black Panther Party’s Community Learning Center in Oakland as a teen. She’s one of the most progressive Democrats in the House.