Not sure if I should answer as I am a few years removed from being a teen....
But in general--I have cruised at ages 9, 15, 19, and my honeymoon.
Our family loves cruising and we would always make the best of it. Parents let us roam freely--something we could not do at home...super late curfews (like 10 or midnight), better than at home, food everywhere, and we were into the cheesy bingo games and of course the shows.
Had
DCL been around when we sailed at my age--my mom would've taken us in a heartbeat and we would have LOVED it--and I haven't even been on it yet.
Of course at 19--it was back when 18 and up could drink on the ships in international waters--but I think all the ages are now 21 (I know for certain on the wonder). But I had the stern parent lecture--and only had maybe 1 drink in 1 day--if at all during the cruise.
At 15--I had met a buddy who was one of my tablemates s well as my then 11 year old sister. At 19, I had my then 15 year old sister and 13 year old brother. I think as long as you have someone to hang out with--it should be okay--but if you are the solo kid in your group (or there is a big age difference between you and a sibling), then that could make a big difference. Meeting a friend on board could make a huge difference in your enjoyment level. This is I think for any cruise line. I went on cruises at times when they did very little for kids/teens and they just do soo much more for you guys now.
No reason to be scared. AS with any adventure in life--it is what YOU make of it. If you think you will be bored--it probably will be boring...but if you have a hand in picking your port excursions--figuring out what on ship you would like to do and make the best out of those opportunities--then you should have a great time. Me and my sister--preplanned our cheesy bingo games, pool time,slide time, dance club time (in the clubs we were allowed in), show time et cetera.
Perhaps posting questions on the activities that interest you--might give you an idea of what to expect.
Aprilgail--did the 15 year old in your group not click with anybody--or is she an introvert or any other underlying factor to why you think she didn't enjoy the cruise?