I told my girls that they would earn that privledge at middle school, along with a cell phone. IF they deserve it, and only if. If not maybe they are not old enough and will have to wait another year. I also told them that it was a privledge, not a right, therefore, I will set it up with them, I will know the password, and that they will treat it responsible, because they never know when I will go through and read it to see if something inappropriate was happening. I think I have only checked it once in the year she has had it and that was because someone said that she sent something inappropriate to another girl. (Which she didn't, the girl sent it to her and I was able to delete it before she ever saw it, TG)
My girls are really good girls, but I want them to stay that way. When they get in trouble, they lose their computer privledges for the day/week etc. depending on what they did. The world wants them to grow up so fast and they will have plenty of time for all of that grown-up stuff soon enough.
Good Luck!