If your passport or birth certificate say you are 21 you won't have a problem purchasing liquor onboard
The sail & sign cards are indeed coded if you are underage, and you won't be able to purchase liquor. If your parents use their card to purchase the liquor I suppose no one will be watching close enough to check. (But I'm guessing they wouldn't be doing that
Some of the island stops have less severe restrictions.
Frankly.. though it probably sounds "preachy" to you, you're young enough to have lots of time to consume liquor. It really is just as easy to have a great time without the booze.