On royal, they can technically be any age, but no youth staff will advertise that and they only really suggest it for voyagers (9-11 age group) but you have to allow it and put it on their from, and then they get put on a separate list, and some ships give a self signer wristband too so it's obvious to the staff. But self signer only works when the ship is at sea/moving.
Any age can check any age out too, but again only when the ship is at sea.
When the ship is docked, someone over 18 has to pick up and drop off.
As soon as the ship starts moving they can check in/out alone again.
Sometimes that can be mid session, so they can come in in the morning and it's a late docking and then they get stuck and can't sign out by themselves, or they can't check in But before the end of the session the ship starts sailing and they can leave by themselves again.