Again, I'm a proponent of every 18-year-old having the ability to drink, or no 18-year-old being considered an adult period...and I tend to lean towards the letting 18-year-olds drink side.
That said, if your concern is some dependent kid getting his buddies drunk, and that's the reason people argue against allowing military members to drink on base...the simple fix is that only people with a CAC card, not a dependent ID, can buy alcohol at age 18.
Why can't the regulation be that restrictive? It's already been pointed out that drinking age on base is a WG/CC decision anyway. He could just as easily say no dependents can buy alcohol on base, or no one wearing a blue shirt can buy beer, etc. It's not a law, it's a local policy.