"From 1968?"
Not sure, I have an international drivers license and I don't see an epiration date on it.
Licenses from foreign jurisdictions are only valid if you are a "resident" (as defined) of that jurisdiction.
See sections 12502 and 12505 of the California Vehicle Code.
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=veh&group=12001-13000&file=12500-12527