last year we cruised twice, DS was 8 the first time and was a month past his 9th birthday the second time, and we gave him the sign in and out privileges both times, no issues arose. He was on his 3rd & 4th disney cruises, knew his way around the ship and had studied the deck plans w/me before the cruises. he was allowed to go from the club to the stateroom, though he could then leave a note and go to the drink station or scoops or plutos on deck 9, or to the basketball court, or to quartermasters, likewise with a note. and after going to one of those places he listed on a note he had to return to the stateroom. or he could go back to the club and check back in and leave us a note telling us that, too.
he is well behaved, never caused any trouble for anyone. we talked during the drive to the port about situations -- like he was to assume that anyone who approached him and said they were sent by mommy or daddy was lying because mommy & daddy didn't know anyone on the ship and we wouldn't send anyone else for him. he was never ever to go into anyone else's stateroom unless we took him there ourselves. we talked about identifying who the cast members were, and how to go to them if he felt uncomfortable about anything (go to cast members from the club, or at guest services, or an officer, or even a restaurant host/greeter maitre d') someone else was doing. never lean over a rail for any reason whatsover--even to view a dolphin. no running. that when in doubt or confused, the stateroom was where we would be returning when all else failed and no one else is allowed in. and if someone was grabbing him and trying to do something against his will, to scream as loud as he could "help help help this stranger is hurting me!" we always told him where we would be -- we really didnt ditch him much, maybe to go to quiet cove for 90 minutes, or one evening in Palo. he didnt care for much of the lab programming, mostly enjoyed playing PS2 on the big screen in the evenings. a few nights we went to sleep and he let himself in.
but we have a very responsible mature child. i have a nephew 5 yrs older than him to whom i would not give such privileges--so you have to know your child.