If you yourself are under 21, you won't be personally allowed to bring wine through customs but you can certainly give it to your companions who are of age. I have to admit, too, sometimes the customs agents either don't notice or look the other way on the DOB thing for alcohol--- I was 18 for my first trip to Mexico and brought back a large quantity of booze. As for amounts and duty, it depends on where you are coming back from, to some extent but if you ask the FA's on your outbound flight, they will probably be able to give you all the details.