This is a very difficult type of attack to defend against for a couple of reasons. From my point of view (70 years old, carry always whereever it is legal) being in their preferred demographic, and aware of the phenomen, it's still nearly impossible to defend against because the response time window is so short and the number of attackers.
My definition of the response time window being the time between when an attack is certain enough to justify the use of lethal force and when they land the first blow. If my hand isn't already on the gun, getting it out in time will be very difficult.
Respond too early and they cry to the authorities that they were attacked by a crazy old guy, too late and they've landed the first blow which, for senior citizens usually means they've already won. My body doesn't deal well with being hit, and it doesn't heal very fast. I'm active and in good health but hit me really hard and I'm probably permanently disabled, or at least won't recover completely for several years.
Second issue to deal with is how many there are. Attacked from several directions at once all one can hope for is if it's possible to get off at least one shot, the rest of them scatter. If they don't it will be difficult to prevail with out serious, probably permanently disabling, injury. Near as I can tell being able to put one's back against a wall, or against a van in a parking lot to control access could help, if that's possible. Attacked in the open, or in a parking lot by two or three hiding behind cars, it's going to be impossible to control access.
Those saying situational awareness is the key are absolutely right. But it may be difficult to detect the attack in time with absolute certainty.
Clearly one key to a good defense is to carefully choose the times one is out and about. I need to find and study the reports of attacks to see if a pattern emerges that allows choosing good times to be in store parking lots, etc. At this point my wife and I already do not go out after dark if we can help it mostly because we don't see well at night - there is age induced deterioration of the rod cells that reduces the ability to see at night.
The best defense might be to go out in packs of armed senior citizens. Four or more of us in a group with the strong hand on the gun in a coat pocket can cover the 360 degrees better and have a better chance to respond.
One tactic that just occurred to me would be to drive to the front of the store to let my wife out with ony a few feet to go to be inside. Then wait in the car till she calls me on the cell to say she's ready to leave, drive to the door, load the car right there, and then leave.
Doesn't work well for some situations, like when we both want to shop together. For those situations picking the right time is about the only defense. Summers may be worse during the week because they aren't in school.
Fitch