18 is the magic age for clubs and adults areas. Disney follows US laws for drinking age, and it is 21. And the policies seem to be well enforced. A server can tell just by looking at your card that you are, or are not, 21. A funny story....I have been aboard the Magic with my daughter several times, both before and after the magic age. One cruise we were in one of the clubs, and I happened to have three KTTW cards in my pocket. We (I) ordered alcoholic drink, and whipped out my card to sign for it. The server flirted a bit, beating about the bush, trying to decide how to handle the fact that the sweet little lady was NOT 21....I had hauled out Meg's card by mistake. So they do look for the age.
When we cruised on the original 4-day Magic, when she was 18, she could drink while we were in Nassau.
Marilee