I think people are a little more edgy than they used to be...
An example: The Clackamas Town Center Shooting that took place in Oregon last year was posted all over 4chan prior to it happening, had anyone taken him seriously at least 2 more people would be alive today(excluding the shooter).
Honestly in a situation like this here is what I think should happen:
If you think it is genuine threat, sure detain the kid. Run a background check and have it go through a threat assessment. If he passes everything then let him go.
Some people have such a detachment between what happens on the internet and what happens AFK. It is only a matter of time before people are forced to realize that you shouldn't say anything online you wouldn't in person and this is a fine example of that.
I hope the kid has learned to filter a little better in public settings and that his life is not impacted too much by this unfortunate event.
Your computer is not a suite of armor and there is no such thing as true anonymity, sorry guys.