Shotguns make great HD weapons for those of us that practice with them. Misused, they are egregious in those situations. Most oldsters resist the idea of practice, and anything that kicks will not be practiced with.
Went through this in the early 80s with my mother's mother, as opinionated and miserly as any woman out of Sicily. She wanted to learn to shoot becaus ethe 'hood was changing and not for the better. She refused to get anything decent for a house gun, but insisted on using her father's top break, nickle plated Iver Johnson 32. I did treat her to new ammo, the stuff she had went back to Prohibition.
A gunsmith friend took it apart,cleaned and lubed the insides, and advised me that it would go bang on command. I tried it at the range and kept them on the paper at 10 yards, and took Gram and her sister to the range. Both ladies were in their 70s at the time, arthritic, and as pigheaded as I am. But, after a trip to the range or two, both could hit a volleyball every time at room range. They pronounced themselve adept, refused to practice, and considered themselves ready for trouble.
If your grands or other seniors are not as obstinate as mine, try a 410 like that little Mossie(W/O the PG). Make sure they practice with it.
Even a 20 has more recoil than most tyros like, and seniors do have limited upper body strength and varied health probs that make a shotgun possibly not the best choice. Go carefully here...