That's were I disagree.
Arbitrary age designations of behaviour in based in law are often based on nothing but the biases of the judges who made those rules.
Example: In the USA drink alcohol age is 21 but you can join the military and kill the enemy (people) at 18. WTF!!!
Age of consent in Canada is often 18. I disagree with that arbitrary number.
In my own profession I will deal with/treat anyone I consider competent under 18 ( I get a bit squeamish around 12-13 years and under).
I've dealt with more than enough 15 year olds professionally (and 40 years ago) that I do not consider any of them "children".
The law is faulty. 15 year old are very capable of making their own decisions and therefore accepting consequences.
So for poor, dear Omar:
He is not a terrorist.
He is soldier, like any other. NOT a child soldier.
But his side was pro-Al Quaida/Afgan/Islam, anti-western/freedom.
I am no lawyer but it feels like he is traitor not a terrorist.
So treat him like the traitor he is.