I recall hearing Massad Ayoob say that in the case of an older victim being attacked by a younger, fitter/faster assailant, the "disparity of force" of the situation gives the victim the right to use lethal force - even if the assailant is unarmed. This also applies to women being attacked by men, or a victim being attacked by multiple assailants.

You prepare physically, the best you can based on your health and abilities, but by all means protect yourself as the situation dictates! As you said, one of the benefits of age is that it normally brings wisdom, and hopefully, enough sense to avoid doing something foolish - like shooting someone for texting during a movie.

I'm 58, and earned advanced belts in two martial arts (even started Krav Maga last year), and while I feel it's important to know how to do MORE than just shoot acuratelly I'll be damned if I'm going to refrain from shooting if I believe the situation calls for it.