For military personal
For military personnel. The state laws were changed to 21 when the Federal government threatened to remove highway money. I think Louisiana held out for quite some time.
If someone can't wait 3 years to drink (18-21) they don't need to be drinking because they don't have the maturity (patience) for it. Getting wasted is not a right nor a necessity. If people were reasonable we wouldn't have to worry about it but people are not able to control themselves.
We are talking soldiers here. For the general population, we're talking about people that can enter into contracts, pay taxes, can get married, well past the age of criminal responsibility, can operate a motor vehicle, can purchase a firearm, and of course, can be drafted into the military.
At 17, you are eligible for a security clearance in the US, up to TS. I know that all rangers have secret clearance, and with recent missions, quite a few are TS as well. Most rangers are in their early 20's with quite a few in the 18-21 bracket. We trust these people with national security issues, not to mention the lives of their comrades. However, they can't be trusted with alcohol.
The same goes for our police forces, firemen, and public servants.