I do not drink any more (and have not since I was 22 or so- sometime early in the Navy, but before I was deployed), given that it effects my physical proformance for a couple of days afterwards (I was training for a marathon at the time and now for the last couple of years I just need every ounce of energy that I can get). Plus, I feel it is unwise for me to drink with meds that cause damage to the liver.
I would say if you are bound and determinded to do it (which I would be if I was turning 21), go with something such as a frozen hurricane (like an alcoholic slushie), and just ask them to mix it very weakly. And if you feel like it is effecting you, then just ask for a non alcoholic one. After all you can also just drink pop and say it is a rum and coke.
If you want to go with bottles for safety reasons, I would stick with the fruity ones, but that just use to be my person perferance. Just research which one's have the least amount of alcohol in them. But bear in mind I stopped drinking because it effected my athletic proformance, long before I ever was dx with crohns.
Since this is a highly individual thing I think take it easy and just going slow might be something to consider.