I know this thread is old, but I wanted to make a point.
Society needs to get over this "innocent teenager" mythos. I am not speaking specifically about this case, but it is a good example.
A group of unarmed teenagers can easily present a deadly threat. Heck, one big teenager can easily present a deadly threat. 6'2" 210 is 6'2" 210.
Deadly force consideration must include the totality of the circumstances, however...
If I am confronted by 2 or 3 or more "teenagers", armed or not, you can bet that I would be very concerned for my safety. If my wife is with me, that concern goes up a notch. I am not as fast or strong as I used to be, despite my training, and I know it, and my wife never has been.
If I am responsible for her safety, then the stakes are higher and my assessment will be adjusted accordingly.
A group of unarmed teens attacking or threatening to attack (encircling, blocking escape, lanquage, posture, attack cues, etc.), as in a robbery, could easily constitute a threat that would place the reasonable person in fear for their life or in fear of serious bodiliy injury.
These "teens" are known at times to beat people stone-cold-to-death.