To review: you asked, "Is it wrong to slit the throat of a 16 year old boy who is napping, if you are on your way to kill a terrorist?"
On the face of it, you are asking whether it is wrong to intentionally kill a person who is, atr the time, helpless, and who is not aggressing against you. (What you're on your way to do, is irrelevant; the boy's age is irrelevant; and in your description there's no reason to think he is an aggressor.)
It's a peculiar question.
Slitting the throat of a sleeping adolescent is not an act of war at all, let alone an act of "just" war. It's merely murder.
Even if the sleeping person were a IED-planter, the fact that he asleep would presumably mean you have a way of stopping him without killing him. You have a moral obligation to stop him. Tie him up, surely. Throw him in a locked room, surely. Break his arm, possibly. You do not have a moral right to slit his throat.
A sleeping 16 year old shepherd boy guarding the yard outside of the building of his leader, who you need to snatch, for interrogation.
You should be training our Special operations people, you could explain to them that slitting the throat of a sleeping sentry isn't necessary, that you could just wrestle him down, break his arm, and carry him with you, until you can enter the enemy's headquarters and place him a locked room.
You don't seem to have any grasp of what the military has to do, it gets far worse than killing a sleeping sentry, you must not know that soldiers sometimes have to execute all of the enemy wounded, and in a worse case scenario, kill one of their own who is injured and can't travel, or even a young child who has seen them deep behind enemy lines.
If you think that is bad, then you sure don't want to contemplate bombings and artillery and Armor use, in city fighting.
Do you know how our soldiers cleared a house in WWII, not knowing if it was empty, or occupied by a family, or by enemy?
Ether we go passive and stop resisting, and surrender, or else we have to do things that you evidently, were not aware of.