Well, you know how this affected me; there aren't many Snarrys written about an older Harry and Severus, and not one that I know of that addresses this subject of how they might handle 'decline.'
What you've done here shows that you're very knowledgeable about it, and have thought a great deal about it too. The scenes between the two of them were really heartwarming (they clearly loved each other very much, unabated by their advanced years) and heartbreaking (I think it's the inconsistent awareness of what's happening to oneself that is most painful).
It made me think about different cultures and how they handle this issue. I'm so very glad their children were good ones, especially David. My favorite part, though, was when Harry spelled it out, how although they were old, it was their magic still that was keeping everyone safe. I liked that reminder; it's a good one for all of us who look at older people and sometimes that's all we see; all of them had a 'younger' life, where they were passionate about something, loved and were loved, and they're still those same people on the inside. I liked the reminder that Severus and Harry still were powerful and important.
You did such an excellent job with this, R., really, and I know it's a story that's enriched me, that I'll think about in the future, and which has reinforced my belief that age is truly a state of mind, and nothing more. And that no matter what happens to the body, or the mind, the soul deserves dignity.