If an 18 year old is considered adult enough to vote or go to war they should be allowed to consume alcohol.
It might be a reasonable compromise to allow those who serve, and are under 21, to consume beer or wine, on base, but not restore the 18 year old age limit for the general population.
You could use that logic to stop people buying guns on the grounds that some will abuse them. If you are 18 years old and are old enough to vote and go to war, you are accountable for your actions, and if you abuse the privilege, you pay the price, just like everybody else. In the UK, drinking is legal at 18, and drinking with a meal is legal at 16. It makes sense to introduce it slowly so that the novelty has worn off somewhat by the time they have reached adulthood.