We cruised on the 7-night Disney Magic Eastern cruise during our spring break in 2002 (late March).
We live in the Chicago 'burbs, and our spring break in 2004 will be the same week as yours. And we'll be be on your cruise!
It seems the public and private schools in the US have spring breaks throughout March and April. In many areas, spring breaks are still the week before or after Easter. In 2004, Easter will be on April 11.
I can't answer the question about "more teens," because I really didn't pay attention to this, nor do I have any insight into how spring break weeks compare to other weeks in this regard.
With over 2 thousand passengers on board, there will be plenty of passengers at every age, from 12 weeks on up.
One comment about teens: One night we were in the Buena Vista Theatre for a 10 p.m. movie. A few minutes before the movie started, about a dozen teenage girls entered and all sat in the same row, a few rows behind us. My wife and I looked at each other. We each knew what the other was thinking. But we were wrong! The girls were quiet and well-behaved throughout the movie.