Here's a bit of nerdish legal stuff I found. It's just "in general" since there are so many gaps in what we know about this case (for ex. if AN ran away or was abducted; AN's age when moving in with ED, etc.) Just thought it might be of interest to some:
It looks like harboring a runaway under 18 is illegal in both AZ and MT. Sources I found said it was illegal in "most" states but then I couldn't find any states where it would be legal. Some possible exceptions to the law were noted though, such as (possibly) if the parents refused to care for the minor child.
Laws vary some by state but from what I found, harboring a runaway generally seems to be a misdemeanor but a serious one, just below felony level.
The police might not bother pursuing it if it's in the past though, if they don't think it's worth the money and effort needed to prosecute for it.
There also appears to be a possible loophole, in some states, about if the adult harboring the runaway knew the runaway was a minor or not.
And there also appears to be a statute of limitations on legal action for harboring a runaway. The one I found (not sure which state) was two years from the runaway's eighteenth birthday.
Another possible issue is age of consent (for sex). In Arizona, it's 18 but it's only 16 in Montana. My guess is the age of consent in the home state is what would count, if a minor isn't supposed to be out of that state in the first place. But I couldn't find a solid answer about it.
All MOO and/or subject to correction.