A cruise at that time of year should have lots of young adults your age. Sometimes they will post specific gathering points for teens and do try to treat the post-HS group a little differently. Carnival realizes you don't need/want a bunch of camp counselors.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013