I expect I'll be adding to this after the chat but wanted to get started on my list now. In no particular order, beginning with older protagonists as opposed to discussions of aging (hoping to add some of these as well):
I also wanted to mention that Crossed Genres has a call posted for Winter Well:Speculative Novellas of Older Women.
- Suzette Haden Elgin's The Ozark Trilogy. Magic-working Grannys uphold the social order and dispense wisdom in a confederation of planets modeled on the Ozark culture of the southern Midwest. Responsible of Brightwater, protagonist.
- Robin Wayne Bailey's Bloodsongs (Frost Saga, Vol. 3). Frost is a female warrior who spends most of the first two books as a young woman fighting supernatural battles. At the end of book 2, she settles down, retires and has kids. Bloodsongs has her coming back from retirement as a middle-aged woman to fight her biggest battles yet.
- Lois McMaster Bujold's Paladin of Souls. Ista is a middle-aged dowager queen on a quest to combat a god-driven curse affecting multiple generations of her family.
- Elizabeth Moon's Remnant Population. Ofelia is an eighty-year old grandmother making first contact with hostile aliens on a new world.
- "Thin Spun" by Sunny Moraine in Hellebore and Rue: Tales of Queer Women and Magic Lakshmi is a wise woman, exiled from her people for having loved too well. Intriguing story of intergenerational cooperation and redemption.
- Silver Moon by Catherine Lundoff. Becca Thornton learns to embrace her inner, and outer, menopausal werewolf when she joins the local all-female werewolf pack.
- Terry Pratchett's Granny Weatherwax and Nanny Ogg. The Wyrd Sisters, etc. The two older witches (Crone and Mother) of Pratchett's wild popular trio of Discworld witches.
- S.M. Stirling's Captain Marian Alston-Kurlelo in Island of the Sea of Time, etc. Alternate history with an ensemble cast. Marian is the middle-aged African-American lesbian captain of a Coast Guard vessel brought to an alternate Nantucket.
- Carol Emschwiller's "Grandma" in Report to the Men's Club and Other Stories. An adolescent is inspired to take on her grandmother's superhero role.
- Janet Kagan's Mirabile, featuring Annie 'Mama' Jason Masmajean as an ecological troubleshooter on a colony world.
- Martha Well's Wheel of the Infinite. Maskelle is recalled from a long exile to save her world and the god she serves.
- Ursula Le Guin's "The Day Before the Revolution" in The Wind's Twelve Quarters. Laia Asieo Odo is an elderly anarchist leader whose ideas are about to come into fruition.
- John Scalzi's Old Man's War and others.Military SF series in which characters sign up for the Colonial Defense Forces in their sixties, never to return to Earth.
- Isaac Asimov's Dr. Susan Calvin, robotic expert in I, Robot, etc.
- Vonda McIntyre's Snake in Dreamsnake. Older female healer in a post-apocalyptic world.
- "Fur" by Helen Cross in Wolf-Girls. A different spin on menopausal werewolves (an idea whose time has come!)
- Marge Piercy's Shira in He, She and It.
- Joanna Russ' Abbess Radegunde in "Souls" (Extraordinary People).
- Louise Marley's Mother Isabel Burke in The Child Goddess.
- Lisa Goldstein's Strange Devices of the Sun and Moon.
- Elizabeth Bear's New Amsterdam, Lady Abigail Irene Garrett.
- Terri Windling's The Wood Wife.
I also wanted to mention that Crossed Genres has a call posted for Winter Well:Speculative Novellas of Older Women.