There is a good post about self-defense and less lethal force options for the elderly here: Energy Management I encourage you to read it.

If someone has seen their physical abilities erode through age, illness or accident to the point that they can't deliver enough force to do the job, then enhancement won't be any help.

Better get a firearm.

Absolutely the truth. You need a capacity to do violence to the assailant. The actions of the assailant determines the amount of violence you need to do to him. Old age will stack the deck against you.

The only point I would really add is that the question isn't Firearms or some less lethal tool. The question is about what kind of threat. If they are not a deadly force threat then you can't use deadly force against them. A young assailant punching a young, healthy victim wouldn't be a deadly force threat, but if he was to punch an elderly or disabled victim it might well be.

As an aside - I once watched an older gentleman on the range, his hands were shaking from what I would have guess was Parkinson's but he was still shooting one hole groups with a double action revolver. He could time the breaking of the shot to the shaking of his hands. Apparently he was kind of famous around the range because he had been pulled out of his car by two teenagers and he beat the snot out of both of them. The responding officers found one of the assailants teeth about 10' away. It seems that the Shaky old man had once been a professional boxer.