Are there specific requirements for volunteers (academic, age, physical condition, etc.)?

No, it is not necessary to be a university student or have a degree; the lower age limit is 18 years old or the majority on your country. Concerning physical condition, it is necessary to be in proper shape to do field work in a semi-tropical climate with a rugged geography (hills, ocean, rivers); furthermore there are, at time, outages in the public water and electricity systems in San Francisco.