It depends on what cruise line you are sailing on. I know that if you are on Royal Caribbean and your parents talk to the youth activities leader and sign some forms, you can move up to the 15-17 if you show leadership by attending some of the younger activities for the first day or so.
And just because you aren't in the 15-17 group by default doesn't mean you can't hang with them. On my last cruise there were two girls that were in the same boat as you--freshman who were 14 and they hung out with our group (16-17yr olds mainly juniors in HS). The cruise is only a week so hang out with who you want and don't be shy!
Also, in my experiences on Mariner (3 time cruiser), there was pretty tight security, but there is a back door to the club and you can get in if you are a little younger or older if you just slip in when they aren't looking or if you come in the sliding door with a big mob. My bud (who i met on the cruise and ended up living 20 minutes away haha) and I met these two hott girls who were really fun to hangout with but were 14 and we easily got them into the club.... sliding door is the key. You can also just go through the main front entrance and if you distract the guy checking they can slip in!