Apparently, the previous policy on alcohol, at least for the Navy...did change 8 years ago. Sorry for any confusion - I was under the impression you still could drink under 21, and apparently you can, but the limitations have increased.
I found this at
http://www.chinfo.navy.mil/navpalib/...6/nns96012.txt
The ALNAV implements an immediate policy change regarding
alcohol consumption by members under the age of 21.
Effective April 1, 1996, all Department of the Navy bases
and installations will be required to strictly conform with the
drinking age limitations of the state or country in which they
are located.
This revokes previous exceptions -- most notably
the one allowing Navy and Marine Corps installations within 50
miles of a foreign country with a lower drinking age (i.e.
Mexico) to permit underage drinking at certain establishments on
base.
Overseas installations, and ships or activities visiting
foreign ports, will conform to local country law. But in no
case will drinking be permitted below the age of 18.