For our first cruise my wife and I were a little scared to do a long cruise. What if one of us gets sea sick easily, or can't stand being cramped in a cabin, or we get bored. So to play it safe we chose one of the shortest cruises we can find. Being in SoCal we are also lucky that Long Beach is a short drive away.
In any case we chose one of the 3 Day Baja Mexico cruises - known for college kids wanting to get drunk in Mexico. I guess if we had a choice between a ship full of college kids or a ship full of retirees we'd rather have the college kids anyways.
Turns out the ship had a good mix of people with families, college kids, older people, etc. Probably on the younger side I would say judging from some videos I saw of the recent cruise in Norway that got disabled and passengers had to be rescued with helicopters off the ship.