One representation doesn't create the validity of any argument, nor does it have any significant effect on the statistical probability that any particular gun owned by any individual is far more likely to be used to kill or injure a person other than an intruder. What this does do however, is provide evidence that one of the major talking points on the liberal side is in fact false, guns can, and in this example do provide security and safety for owners. That popular narrative, that guns (particularly certain types of guns) have no redeeming qualities or legitimate uses outside of a military setting is patently untrue, and should be called into question whenever it is applied. This example shows exactly how such a weapon can be used effectively. The person felt threatened by the armed intruders, responded appropriately, and protected his own and his loved ones' lives.
The argument isn't that those under 21 don't need to defend themselves, it's whether or not a person under 21 can make sound decisions. This is hugely problematic for me from a logical perspective. If we as a society deem a person not old enough to make important decisions until age 21 (a position which is not inconsistent with psychological research incidentally), then we should not deem them eligible for military service either. Currently a person can voluntarily sign up to serve, or be required to do so (drafted), either of which arguably can require death or killing, at age 18. Either a person is an adult at 18, can presumably make good decisions, and we then extend to them all the rights and responsibilities of society, i.e. voting, alcohol, guns, driving, military service, tobacco, or they are not, cannot be trusted to make these decisions for themselves, and these milestones are put off until age 21. We can't have it both ways.