I can't quite remember the title, but there was a recent trad novel published where "the chosen one" was a grandma in her 60-70s. Tried searching it, but memory's cloudy. Darn. Cosplaying as a geriatric pretty good today.
There's also Michael Chabon's
The Final Solution which is basically about an elderly Sherlock Holmes, without explicitly being stated so.
You can also try
Remnant Population by Elizabeth Moon. Haven't read it, but it's on my TBR list. It's basically a mix between
The Martian and a first-contact story on a faraway planet. With the protagonist being an elderly woman, who decides to stay behind and live out her days in the place she calls home, while all other colonists leave for plot reasons.
Alternatively, here are a couple of lists to sift through (external links):
https://www.reddit.com/r/Fantasy/comments/w1zdiz
346 books based on 163 votes: The Legendary Wolf by Ammar Habib, Like a Mule Bringing Ice Cream to the Sun by Sarah Ladipo Manyika, Water for Elephants b...
www.goodreads.com
Don't know any web novels that fulfill your requirements, sorry.
