Why would that not hold up in court?
Various reasons because of the age.
The spirit of the law versus the letter mostly as discussed in several articles concerning the case.
Kyle is old enough for military service and hunting. (Well regulated militia)
But the Wisconsin law was written to keep firearms out of the hands of juvenile gang members. So that they could prosecute juveniles that were intent upon committing adult crimes.
So...there's the problem. No one questions why Kyle was there. That piece of evidence is undisputed. (Motive is clear) Kyle was acting in a defensive and not offensive fashion. (Being part of a militia that was trying to save businesses from riots)
The incident surrounding Kyle seems to be initiated when Kyle extinguished a dumpster fire the the now deceased started...a fight ensued for the firearm he carried and the results kept escalating conflict.
The rioters were carrying weapons ranging from a makeshift mace to clubs and firearms.
And this is what is really everyone's question. The charges won't hold up for a large variety of reasons (too specific in a grey area of law that was never intended on being used this way) ...but the secondary questions will remain.
When the police, in essence, give up fighting civil unrest is it legal for a private militia or individuals to act on behalf of the public or person of need?
We have a lot of "Good Samaritan" laws that would suggest that they should...and in fact there's several laws requiring off duty professionals to do just that.