Welcome back, Melissa! I'm laughing that you said your 17-year old might treat his friends to smoothies on your account, because I can see my son doing that too, and he's only five. Mr. Generous, that little man of mine!
Though my kids are younger, so I don't give them charging privileges at all, I do understand that you'll want your teenager to be able to make purchases. I called Disney Cruise Line to check in about this, as we've always had just one credit card on our account. I found out that you can set up a different charging account for you son.
When you complete your online check-in (75 days prior to sailing for
first time cruisers), you will enter your credit card for incidentals and select who has charging privileges. Just leave your son off that list at that point.
Then, onboard the ship, head over to Guest Services (though you might be able to do this as you check in at the port too, you might ask), and one of the Cast Members there can take your son's Key to the World Card and link it to his debit card for charging purposes. Now he can treat all his new friends to anything he can afford and not surprise you with a bill full of smoothie purchases!
Pop back here with any other questions you have, Melissa!