Why the fuck would a 80 year old be walking down a dark street alone?
I could counter with the obvious "Why shouldn't they, if they want to?" and point out that a member of the previous generation of that family was still happily and capably walking to visit friends or go shopping at nearly 100, but that doesn't really get us anywhere.
In the specific case I had in mind, I'm talking about the oldest member of a family walking back with the rest of his family to their car, after visiting my family.
That person is perfectly capable of getting themselves to the car without needing help from anyone else, as long as they can see where they are going. In fact, as a matter of independence, I'm quite sure they would want to do it themselves. Most people I know of that generation who are still with us take great pride in maintaining that independence as much as possible and not becoming a burden on others, and I firmly believe we should all help them to do so for as long as they can for basic quality-of-life reasons.
Of course their children would help if necessary, and so would anyone from my family, and so would other neighbours if they saw there was a problem. No-one here is suggesting leaving an octogenarian in difficulties to fend for themselves. I'm just saying they shouldn't be put in those difficulties in the first place if it can reasonably be avoided.
Turning off the lights has a disproportionate effect on older people -- not just octogenarian kind of older, but also drivers or cyclists in say their 50s or 60s who would routinely travel independently and probably wouldn't describe themselves as old, but whose eyesight will nevertheless be far less effective in the dark than it was in their twenties. The cut-off point will be different for everyone, but at some point the effect will be enough to make people who would otherwise have felt confident going somewhere not to go out any more, and I don't think that is a good thing.