I'm not sure if it fits well with the other books in this list but I absolutely love the horror novel The Haar by David Sodergren. It's a mix of cosmic and body horror, but the main character is an octogenarian and her similarly aged friend. It's actually a really sweet story wrapped up in a horrifying blanket.

i recommend vladimir!!! protagonist is a 50-something year old woman

Ooo I love this. Might I recommend Death in Her Hands by Ottessa Moshfegh? I can't remember the MC's exact age off the top of my head but she's described as elderly.

If you're taking recs, maybe The Wall by Marlen Haushofer!