I think it is fair to say that on the whole literature is more interested in exploring the climb into middle adulthood (and whether the core characters "put away childish things" and gain maturity and self-awareness) or even late adulthood and generally avoid looking too closely at the 7th age or rather a second childhood "sans teeth, sans eyes, sans taste, sans everything." I broadly assume that it is just too depressing to face this decline head on, as well as the fact that it just takes a lot of energy to write a novel, and most novelists hang up their spurs somewhere in the 6th age.
That certainly doesn't mean that there may not be portraits of adults taking care of their aging parents, but there are not that many novels where the central characters are quite old and generally in decline.
Off the top of my head, I can think of a few exceptions: Bellow's Ravelstein, Heller's Portrait of an Artist, as an Old Man, many of Alice Munro's characters, Molly Keane's Time after Time, Fuentes's The Old Gringo, Oscar Cesares's Amigoland and Garcia Marquez contributes Memories of My Melancholy Whores and The General in his Labyrinth. Of the poets, Adrienne Rich, Charles Reznikoff, Earle Birney, George Bowering and arguably Sharon Olds come to mind. Here are a few more, though I am not sure I would really count Spark's Memento Mori, since she was only about 40 when she wrote this, so it is very much a theoretical exploration of growing old (and death).
The General in his Labyrinth may actually fall into the category of someone in the 6th age (who can still largely take care of him- or herself) peeking over the edge into terminal decline. This may actually be the most upsetting stage of all, and it isn't that surprising that is largely met with psychic displacement or outright avoidance. Another few works that follow this pattern are Last Man in Tower by Aravind Adiga, Philip Roth's The Humbling and the play The Dresser by Ronald Harwood. Emily, Alone by Stewart O'Nan puts on a positive spin on this stage, and the priest who narrates Gilead by Marilynne Robinson has also made his peace with growing old and knocking on heaven's door. I'm not sure whether Harry Angstrom from Updike's Rabbit at Rest would count. My understanding (I haven't yet* found the time to tackle the Rabbit series) is that Harry is more on the edge between 5th and 6th stage, worried about being on the downward slope -- and indeed he didn't last all that long once decline set in.
While a few authors play up the infirmities of age for laughs (I'm thinking primarily of Molly Keane), this is a subject that is quite depressing and makes for tough reading. In a way it will be interesting to see what happens as a large wave of American writers from the Baby Boomer generation keep writing well past retirement age. The confessional urge that drives many of them will likely run up against the wall of indifference (and avoidance) of a culture that worships "youth."
I'm not thinking of these issues so much because of my own infirmities (though probably in 10 years' time I will be hating my current self for not doing more to stay in shape and stave off decline) but because I have been reading Rohinton Mistry's Family Matters. The first 100 pages are grim indeed, as Mistry introduces the retired professor Nariman, who is suffering from Parkinson's, and shows how his step-children (themselves nearly of retirement age) cannot cope with caring for him. While the novel does lighten up a bit after this opening, I am still struggling with it and can't imagine reading it a second time. I guess I'm not quite ready to meditate on the 6th and 7th ages of man right now. More escapism, please...
* Indeed, I feel I really need to read these, so moved them up quite a bit on the list, though I still might not get to them until 2019! Then I slipped some John O'Hara into the slot that had been occupied by the Updike.