You know a fun fact that I just learned recently (although I'm not sure if it's true so help me out)?
Apparently in the US, it is only the sale of alcohol to minors and the purchasing of alcohol by minors which is illegal. As well as public possession of alcohol by a minor. So it isn't technically illegal to drink in my own house? Not that I ever cared when I did, anyway.
But anyway, like mr. dave said - what's to stop people from just mixing them? Although I guess it's less of an encouragement to do so.