I think each parent can assess the "feel" of the shipboard community and the ability / maturity level of their child and make a judgement about what is appropriate. Not every child will fit into a "schedule of readiness".

I allowed my 15YO DS complete freedom on a Carnival cruise. He was told to be back at the cabin in enough time to prep for dinner - I expected to see him seated in the chair when dinner began. On sea days, that might be my only sighting of him awake! He would be out an about with other teens until all hours and sleep till noon. He was with us on all port excursions, of course. I must admit, he did have more freedom on board than would be allowed at home - definitely he would have a curfew at home, for one thing. We got to know some of the other teens - and their parents, and the flock of kids would make sure to cruise by the parents at least once or twice a day, just to let us know they were ok.