How old is old enough?

I got one of those phone calls yesterday that every parent dreads. The school assistant principal called. My 11 year old 5th grader was in trouble. His crime? He visited an unapproved website while at school. The website in question www.google.com. He and his friends were looking up Boa Constrictors. If he was in 6th grade, he would be have been allowed to visit google. K through 5 are too young for any website other than the 3 websites that the school approves of. He received a day of in school suspension. I understand the need for this and have no complaints about it but...

This got me to thinking, how old is old enough? 

The same day that this happened, my 13 year old, 7th grader wanted to know if he could start a facebook page. My rule has always been not to allow things like this until high school. No email, no facebook, no myspace...nothing that can be identifiable to a particular child. I want to know that my child will make good choices on his own before I allow him to be in that situation.

After much discussion and thought on the matter, I finally allowed him to start a facebook page and email address. My husband assures me that 13 is an okay age for these kinds of things.  I will be vigorously monitoring all of his online activities but I'm still very torn though. Is he really old enough? Should I have made him wait? 

The hard part is the knowledge that you can't "unsee" what you saw by mistake. The internet is a great place as long as you are careful. 

When it comes to the internet, how old is old enough?