As a parent, it is my RESPONSIBILITY to say "no" when my daughter wants to engage in adult-type behaviors when she is only 13.
Just curious, what adult-type behaviors are we talking about here? Taking the bus alone, throwing back a fifth of scotch, dating?

I'm a big believer in you've got to give them room to make mistakes they can learn from, and letting them be accountable for their own choices.

Computer and things like private facebook pages do complicate things. I'm pretty sure that if I had non adult children I'd want to check in too... too many kids do have poor judgement/lack of understanding about things of a titillating nature , and our culture doesn't help it.