Its not about the age its about them knowing who you are.
In Canada I was even told this buy security at a outdoor concert beer garden.
Everyone must have 2 ID's to get in even if you are a 100 years old.

They claim the "Act" was created to ID gang members in nightclubs to stop gang violence.
I have used non-government issued ID to enter nightclubs that had no date of birth on it and it was accepted.
A few times I got in by just saying I don't have any and I stand staring in their eyes.

In California I had store mangers refuse Canadian government issued ID for beer.
Claiming only California issued ID is accepted. A joke. These people are close to retarded in IQ's.

18 was the legal age to drink where I was born.
What gets me with the 21 year old age limit on drinking in the US is that you can join the army and kill people years before you are allowed to have a beer. That is very disturbing.