I sympathize with this kid, not that I endorse under age drinking but half this board is about getting cheaper drinks/sneaking booze on board. You can't tell someone drinking isn't worth it when its everywhere and everyone does it and is a huge part of cruise life for the majority of cruisers..
And to answer your question: No you can't buy liquor on board but I'm sure you can acquire it if you really want to. Just be safe where you drink and with who you drink-we obviously can't stop you. So be smart about it