I've been on three cruises, the fourth coming up this August. Two have been Carnival and one was a Princess Cruise.
It's very true that Princess is geared more towards adults; however, that doesn't mean that you may not end up on a ship with a BUNCH of young people. When I went it was right after I graduated from high school five years ago and there were SO many people my age onboard! It was a party every night. Princess is very much a luxury cruiseline though and is BEAUTIFUL! Carnival is definitely geared more towards the younger crowd. The two times I've been on a Carnival cruise there have been so many young people for my best friend and I to make friends with, and the amount of partying on these cruises is HUGE. Carnival is MUCH cheaper than Princess though, so if you do decide to go on a cruise and want a Carnival price but don't necessarily want to be on a party ship, try Royal Carribean. It's comparable in price and geared more towards a more relaxed crowd.
I personally LOVE the format of cruises. I love to travel and want to explore many places. Yes, you're on each island for only a short period of time, but if you set up shore excursions you can do SO many different things! This past cruise we climbed the Dunns River Falls in Jamaica and played with dolphins at Dolphins Cove. This cruise coming up includes stops to Costa Maya, Cozumel, Grand Caymen, and Belize and you'd better believe we're picking the excursion to climb the ruins in Belize!!!!
If you enjoy relaxing with that drink in your hand, catching some sun and listening to music...but also enjoy exploring and checking out new places, I believe that a cruise would be perfect for you! In my opinion it's a great way to mix both kinds of vacationing and I have NEVER had a bad cruise experience. In fact, it's my favorite way to travel!!!