Responding to an off-topic post on the SF Bay Area board:
I believe the oldest restaurant in San Francisco is either Tadich Grill or John's Grill, depending on your criteria.
John's Grill: same location since it opened in 1908.
Tadich Grill: opened in 1849, making it the oldest continuously operating restaurant in San Francisco, but it's been in its current location only since 1967. They brought a lot of the furnishings over from the location they'd been in since 1922, and tried to keep the same style, so it feels very old.
Sam's Grill: started on California Street in 1867, current location since 1946.
Fior d'Italia: started on Broadway in 1886, was on Union Stret from 1953 until last year, recently reopened in the venerable San Remo Hotel.
Alfred's Steak House: opened in 1928, moved to current location (formerly the venerable Blue Fox) in 1997.
Tommaso's (formerly Lupo's): same location since it opened in 1935.
Original Joe's: same location since it opened in 1937.
Jeanty at Jack's is a makeover of the venerable Jack's (same location since 1864).
I've heard the Old Clam House was a stagecoach stop in the 1800s. I wouldn't eat there again myself.
The current Fly Trap is not old, it's named after but otherwise has no connection with the venerable place that closed in 1963.