If I'm going to Hong Kong from San Francisco, what age do I have to be to purchase alcohol in duty free stores?

Since you're buying the alcohol in US, you'll have to follow the US legal drinking age.

When I lived in Hong Kong (1998 - 2001) there was no drinking age. I went to middle school in Hong Kong. Frequently middle schoolers would go out to Causeway Bay, etc. and get drunk.

I expect that much like the rest of China, there is no drinking age, or if it theoretically exists, it isn't enforced. So don't worry about the Hong Kong end.

The issue will be if you bring alcohol into the United States and you're underage. That is illegal. As long as you consume it over international waters, I think you're in the legal clear, except that some airlines bar drunk passengers.

I think that Hong Kong is stricter, but mainland China does not have a age limit.

No nickname - You are very wrong. There was always a legal drinking age. It is 18. Even the time you were in Hong Kong it is 18. They may not check your ID because you look 18 or the staff doesn't care and they just want your money. Don't be stupid, there is legal age in all country in the world. Dumb ***.

No matter if you come from SF or anywhere but if you buy achocal you must be 18,

They are not too concern with your age but your look to be an adult. Anyway, you are Ok so long as over 18 yrs in many countries. If they sell you it is OK but if NO, then you know too. When U can sleep in plane it will travel faster.