If your age prevents you from dealing with the major rental agencies, you can arrange a private rental through craigslist...
This link accesses people offering to rent cars in the entire Bay Area (including suburbs like Pittsburg and Antioch).
Or alternatively, you can compose and run your own advert stating your age and other requirements. I have used this strategy to rent a car one-way from SF to LA, but I did not have an age problem, I was simply looking for a great deal. Within about 1 hour of posting my advert I had enough replies to proceed with comfort.
All craigslist caveats apply.
Also, some private individuals wishing to rent their cars will make an entry on Relay Rides https://relayrides.com/