Why 21? Why not 18? What they are saying is that an 18 - 20 year old that is/was a member of the military and trained how to properly handle a firearm can not be trusted with a firearm when they are on leave, etc.
I searched expecting to find a military wavier. Instead I found that no member of the U.S. Army in Alaska may carry a concealed weapon on or off base by order of Major General Charles H. Jacoby Jr.
http://www.usarak.army.mil/policies/PUBS-ACROBAT/USARAK_Policies/CGCOFS%20POLICY%20STATEMENT%2020.pdf