Having grown kids, I am pretty confident about my opinion on this one. I think it depends on the maturity of the kids and the house rules. For example, the kids were NEVER allowed to have a member of the opposite sex in their room. Like Paul, I also had keys to the locks. 9 is a bit early, but I get the special circumstances. One of my daughters got a lock at 12, another at 16. And my son got a lock when he was 15.