Friend suggestions lets the cat out of the bag

Last night as my wife was cruising on Facebook, a friend suggestion popped up and it was our son. We had recently had some friend's children join Facebook and this is still an iffy subject for us. At what age do you allow your children to enter the online world of MySpace and Facebook? We don't feel comfortable with it yet and had told the children that we would not allow it at this time. Sign up for it and you will suffer the consequences talk ensued. After this friend suggestion we go through all the stages: denial, calling us liars, false memory (I don't remember doing this), guilt with crying and then admission to the crime. The excuse given for this infraction was he was curious and wanted to play the games linked to Facebook. We try and let the children lurk the inter-webs with few restrictions. If we find a site is a problem it goes into the block list on the router and we move forward. I really don't want the hassle of the URL lock-downs within Parental Controls within Windows Vista if we don't need them.
What would you do in this situation? What ages would you allow Facebook and Twitter to become tools for them to use to express themselves but still keep them safe?