Let me take a stab.
If there is not a requirement to have someone older than 21 in the room, and it is only the two of you - both considered adults in the fare you pay.
Some other thoughts:
Drinking alcohol on board - neither is considered an adult, must be 21
Play Bingo - Only 18 year old can claim the prize, although both can play
Teen Activities - Common Ground, Teen Beach, etc. - Only 17 year old can participate
Adult Activities - Spa, Adult pool, Palo, Serenity Bay, etc. - Only 18 year old can participate
IMHO, I would try to go after your 18th birthday. This way you will both be able to participate in the same activities.