I was looking for the definition of "child" in some of those the other day. There didn't seem to be one. The definition of who is a minor may be state law. For instance, it is 16 in Washington, D.C. I still think he is in deep trouble for the communications he did have.
What you have posted may be Federal Law. Foley was working with Federal Laws for Internet Child Abuse and Porn, I believe. Don't know which laws he was breaking, if any, just because I'm not super knowledgable on this.
This comes down to illegal vs. legal, not right and wrong.
What I posted is indeed federal law and as I stated, it may not apply in this instance.
However, there it is.