This doesn't really answer OP's question about "what age," but I did want to pass along some advice. I attended an internet safety seminar at dd's school and the material the police detective shared disturbed me deeply. I will spare you the details, but I do clearly remember him saying at the end, "If you take one thing away from this, please never ever let your children have a computer with Internet access in their room. Even with parental controls. It's like giving an 8 year old or a 13 year old access to the Las Vegas strip and asking them to use good judgement and not explore."
He also made the point that even when kids accidentally find themselves on some of these sites (spelling errors, typing .com instead of .org) the visuals can really impact them because kids are just beginning to understand things like sexuality.
It certainly made us rethink our views on computers for the kids. We plan to make do with the family desktop in a central location for as long as possible.
Of course, ymmv.
