Hi Again OklaCruiseMom,
I took my two kids and their best friends on a Med cruise summer before last. My DS was 17, just a few months shy of 18, but his bf was already 18. My son could go into the Teen Club but not his bf. It doesn't sound like alcohol is going to be an issue for your girls but in the Med for 18 and up waivers could be signed to allow the kids to have beer/wine.
Even though your girls are shy, the odds are they will meet others their age on the ship during activities. My DD and her bf met a lot of girls (and some guys) their age rock wall climbing, tanning by the pool, on excursions, etc. The vibe on ships is different than land vacations, people are friendlier, more open to chatting up others, etc. and kids just seem to find each other. There were days I didn't see my kids or their bfs after breakfast until dinner and then only because they wanted me to know they were going to Johnny Rockets,