When I was growing up I was always allowed alcohol, especially at Christmas.
The first time I was "drunk" that I can recall was at home (not throwing up or anything).
My parents thought was that this would help me to understand drinking, and know when to stop.
Of course I have drunk more than enough at times - everyone has.
I had quite a few friends whose parents were more strict.
This had the effect that as soon as DD or DS was able to go out of the house without parents, they got drunk. very very drunk, and I was left holding hair out of the way or patting backs whilst they were being sick in a car park.
So my personal experience is that if a person wants to teach their kids about safe consumption of alcohol, then they should teach responsible drinking by example.
It's weird, I wouldn't advise teaching about drugs in the same way.
Of course, other people shouldn't be giving your kids alcohol at all, and should at least ask your permission.