I don't think you can really put a set standard on it. Children are different, situations are different. I can't really criticize a wide range of tactics for dealing with filtering content for really young children, though I would definitely support a much more filter-free environment upon a child reaching adolescence.

I'm not sure you can narrow it to a set standard. Or at least, I don't think you should. Parenting isn't about definitive standards that apply to all children.