I'll agree with you on that one. Not many "yutes" have the self-control and/or discipline that this terrorist or terrorists have shown in their escape & evasion.
However, the pure shooting skill shown is not unknown or impossible to find amongst the young.
"Not that of a frustrated teen", probably not. But I wouldn't be surprised if the perps were a couple of thrill-seeking narcissistic older teens or young adults bored with video games.