anon,
The age of 21 is unique to the US, which has the highest legal age in the civilized world. In France, Switzerland, Germany, etc. its 16, and 18 in other countries.
And in the US, there are exemptions for underage drinking for religious reasons (by States that prohibit it otherwise.) Additionally, in most States [including New York State] the law prohibits those under 21 from PURCHASING (and public possession of) alchol, it does NOT prohibit DRINKING alchol by those under 21.