After reading all the posts, I can say they range from amusing to out and out ridiculous. Too many young men have no idea what happens when you get old.
I'm 70 years of age, and my wife is 67. I have ostio arthritis from a life of turning wrenches, and some other handicaps from active duty injuries while in the army. The VA has me rated at 50% disabled. My wife has rheumatoid arthritis that runs in her family. Her mother was very affected by it, as is her sister. With meds, it's somewhat under control, but she has good days and bad.
Neither of us can deal with heavy recoil. We both are life long shooters, and we met on a public pistol range in 1970. So both of us are very familiar with guns. But the last several years we've shot nothing but .22's. I and Karen both shoot Smith and wesson revolvers, as it's easy to load and shoot. No slide to deal with, and if a dud round comes up, just pull the trigger again. A S&W 617 has almost zero recoil, and even Karen's little 317 is managable. There are some older H&R revolvers hidden around the house that gives us 9 shots apiece to deal with whatever. We're both retired, so we're almost always together, and have over the years got some plans of action down pat. With the kids grown and gone, it gets easier as we don't have to worry about the kids and line of fire.
Senior citizens can not deal with shotguns, the wonder tactical auto of the month, or any other macho young mans guns. Time robs you of most of what you are as you age, and most of you are in for a shock in about 30 or 40 years, of what you can't deal with anymore. Old injuries act up, and there's a downhill trend that can be slowed, but not stopped.
These people the OP is talking about need to get a few used but good condition H&R revolvers, practice, and maybe even a couple of .22 rifles like Marlin 60's or Ruger 10/22's, and practice working together as a team. Two seniors, a .22 rifle each and a revolver each, will be a tough nut to crack if they work it right. It's they're home, they know the layout, and they need to practice holding a hallway while one calls 911. The heck with the neighbors on speed dial, make it 911. Again, both need to be on the same page, in the same paragraph, and same line. Unless they've somehow angered a Colombian drug lord or the local Hells Angles, they don't have to be able to repel an armed invasion. Some well used .22's will do well if they have a plan, and rehearse it. A Ruger 10/22 and a 9 shot H&R apiece, will give them 19 rounds each without reloading.
In any real world senerio, that should be more than enough.