All that Mike said plus.....the longer cruises, over 7 days, tend to have more older people as younger ones with families have a hard time getting that much time off of work.
We did the Panama Canel cruise on Princess a few years back. It was 10 days and stopped at Aruba, Cartagena, Panama, Costa Rica and Jamaica. I believe my wife and I, both in our 40's at the time were some of the youngest people on the ship. We had a balcony room and truly loved that cruise.
Go with the balcony....it is money well spent. I highly recommend the balconies at the rear of the ship as it gives you a 180 degree view.
Also cruising while the rug rats are still in school is a good idea too!