“...California was apparently an early adopter.”
Twenty-one was the legal age to drink and to vote in California when I was growing up. Some states had lower ages, but 21 was the standard. When they lowered the voting age to 18 many states lowered the drinking age as well. Don’t know if California did. Then Congress strong-armed the states to raise the drinking age to 21, kind of like they did for the 55 mph speed limit. Do it or we withhold highway money.