Nothing organized specifically for 18-21. You're into the "adult" category at that point, although your seapass will reflect that you're under 21 (on Oasis this means that you'll have the teen silhouette and the adult color - you'll see what I mean once you actually see it).
As for drinking all ships and lines 21+ from American ports unless otherwise dictated on specific sailings, and even if you are 18 and the ship allows drinking under 21, you'll need a parental waiver allowing you to do so.