Welcome aboard, Maria! What a fun girls' trip!
Unfortunately, there is no wiggle-room on the age requirements for the Port Adventures. If it says 18 and up, your 17 year old will not be able to sign up for that adventure. But, the good news is there are many adventures on which she can join you!
For girls that age, I think it's important to mix some history and some fun in the Port Adventures. While Disney does a great job choosing a historical adventures and incorporating fun, you will also want to plan just a plain fun adventure as well.
I'd highly recommend The Best of Rome or Taste of Rome. Both of these will take you through quite a few of the landmarks around Rome and give you some free time as well. The tour guides are incredibly knowledgeable, so all of you will come away from this with a new appreciation for this amazing city.
If you are on the 12-night sailing that includes Ephesus, be sure to take a Port Adventure, because when will you ever get to be there again? All of them are amazing, but The Best of Ephesus, or Biblical Ephesus will give you a look into the history and significance of this amazing area.
Now for some plain fun, how about a relaxing day on the
Island of Capri? What 17-20 year old girl doesn't dream of enjoying a leisurely lunch overlooking the gorgeous water and doing a bit of shopping on an amazing island paradise? And what about concocting their own bottle of French perfume at
The Perfumer's Apprentice in the French Riviera? Can you imagine them sharing that with their friends?
Have fun as you continue to plan and let me know if I can help, Maria!