When I worked at the Cambridge District Courthouse, the age of legal considering was the age of 17. 
Now, with society not caring for their children (such as the lady who just gave birth to octuplets, who already has 6 young children, and lives with mom and doesn't work for a living, but will get aid from welfare and other government system, why the doctors did not already check that?)   it's hard to say.

That being said, sometimes depending on the situation at hand and all of the legal advice that you can get, I think it would go by the crime being done.  I don't think a 5 to 7 year old would have any knowledge about crime, and even if they did, you would have to look at the psychology of that particular child.

But, then again, there are a few teenagers that just don't get it.  They keep coming back to Juvie (Juvenile courts), and keep getting in trouble, as jail or Juvenile system kind of enables them.  It gives them a sense of some weird case of "Home" or comfort.  These kids sometimes come with issues that go beyond the parents or non parenting.

I sure hope that we can all try our best to avoid certain situations and help these kids find themselves to be useful in someway, to make them selves feel good about themselves and not just in the bad areas that they come from.  (See the movie Akyllah and the Bee (sp?)).  I certainly hope that my son learns right from wrong and that he will avoid trouble as he grows up.

Sorry, but that is my opinion....