Depends on the length of the trip - 10 days or more tend to be older because they have the time ... early April will still be spring break time, so younger. Of the 3 lines you mentioned, they'd all be a mix of ages. Party time - go with Carnival ... if you're younger still, try Disney.
I cruise the Emerald Princess, Eastern Caribbean on April 16, 2012
We took our first cruise on Princess. I celebrated my 24th birthday onboard, and hubby was 28. We didn't feel out of place - the ages of the passengers vary greatly. We had a great time....to prove it, we're going on our next Princess cruise next week!