This has changed.... DS 9 and DS 7 checked themselves in all the time. We followed them down the hall and when we rounded the corner, they had already checked themselves in (lightning fast apparently) and were washing their hands. I confirmed with the CM that they could check them selves in, but not out. However, I think my DS 7 was too young for this.
When we talked to the CM at shoreside, they told us that they could have sign in/sign out privileges as young as 8. We allowed my 9 year old (going on 14 and a safety freak) but I can already tell you that my DS 7 will most likely not be allowed to on our 10/20/13 sailing. He will be 8 1/2 at that time, but he's so impulsive, that I can't see him being cautious enough to follow the rules like his brother.