This has never been a concern of mine, but if I remember the speil on the transfer parents can sign a waiver of some sort that will allow their underage children to buy and consume in international waters. Don't know if that caps at 18 or not. Many Europeans think the US drinking age is silly, but this ain't Europe.
I do remember my son (over 21) challenging some other passengers he was sure were much younger than he.
My best advise is to find someone near your age and hang out and have fun with them. I know at 17 you don't want to hang out with your family - how embarassing.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
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Sail Windjammer: Mandalay 2017
Star Clippers: Royal Clipper 2015