According to this Lost Boys Movie URL, "Nathan Smith kindly wrote in to tell us that the 'Lost Boys' cave is also local to Santa Cruz: "It is located off Empire Grade Rd, near the eastern entrance to UCSC. The caves are quite extensive and rumour has it they run all the way to Davenport beach (a.k.a Red White & Blue beach), although we never made it that far!""
We found it using these directions along with a little sleuthwork...