From self-defense to 2nd degree murder with a pull of the trigger.
Against a 65 year old guy, a muscular 17 year old guy (from the picture in the article), even unarmed, is a mortal threat since he likely can beat an old guy to death. If the old guy is in bad physical condition, even an 18 year old girl can be a threat.
Once they're shot and on the ground, though, the plausible threat is greatly reduced. He should have just held them at gunpoint and called police.