Here are some books with older main characters that I've read and enjoyed recently, mainly scifi/fantasy with a pretty good range of characters. Some of these are a little more vibes based as their age isn't explicitly mentioned, but I think they still apply.
Hammered by Elizabeth Bear - Scifi, space exploration, first contact, etc. This is a 3 book series that has aged really well and challenges a lot of character stereotypes. Maker/Casey is an old war dog who's retired to mending kids for a warlord and staying under the radar. Her service left her with an advanced prosthetic, burn scars, and a chip on her shoulder. Her war wounds are catching up with her and her past has come knocking. An absolutely superb near future scifi. Trigger warnings include death of several main characters including a child, addiction, survivor's guilt, terrorism. This is a generally hopeful series with a positive ending, but it has some pretty dark moments.
Ink & Sigil by Kevin Hearne- This is set in the Iron Druid universe and features a older MC tasked with keeping the gods off earth, upholding treaties with various pantheons, and generally helping to keep the secrets of those creatures that belong in myth and legend. Al is cursed and this is the story of how he unravels the mysteries of his curse and ultimately is able to rebuild his life in the aftermath. This is a humorous fantasy series that you won't want to put down that will leave you feeling immensely satisfied. It asks questions about destiny and making one's path in the world and how that changes as you age.
3. A Plague of Giants by Kevin Hearne- This isn't really his usual urban fantasy fare, but perhaps more interesting for it. This is told from a number of perspectives including the old and young. The chronicler is an older scholar recording the perspectives of a number of key figures in an invasion of mysterious giants from across the seas. Prepare for politics, scheming, and human ingenuity, all tied up with an incredibly interesting magic system. The inhabitants can risk their lives to gain blessings that consume life force as the price for miraculous power over the natural world. This is set during wartime with all the usual warnings that that implies, some body horror, and suicide (?). The first book is generally hopeful and deals thoughtfully with the main narrators loss of his wife, aging, and a rapidly changing world.
4. The Prefect by Alastair Reynolds- Tom Dreyfus is a prefect in the aptly named Panoply, which is responsible for monitoring polling equipment and maintaining democracy in its myriad of forms across the Glitterband. This series deals with rogue and possibly insane AI, criminal investigations, and some truly weird worldviews. It also deals with issues of memory, personality, isolation, and technology. Dreyfus's wife is presumed dead at the beginning of the series and the circumstances around her death leave some questions. This is a pulse pounding series about space detectives that has no qualms about toeing the line of an uncomplicated cop thriller and questioning the means and motives of the panoply operatives.
5. The Fifth Season by N. K. Jemison- While the first book is non-linear, this spins a story of a post-apocalyptic earth where earthquakes are common and cold-hearted practicality are key to survival. The story opens with the death of Essun's son and the disappearance of her daughter and the destruction of the world as she knows it. This is a dark mystery that unravels the secrets of the shattered earth through Essun's past and present. Essun has the power and training to potentially save the world or destroy it. Her past will come back to her and she will have to face it to have any hope of finding ending the season, saving the world, and finding her daughter. This is a dark fantasy and not an emotionally easy read, but it is fantastic. Trigger warnings include children's death/murder, abuse (including of children no sa iirc), body horror, depression, etc. If you're sensitive, you may want to find a more specific list of warnings or forgo this altogether, it is not an emotionally easy read.
6. First Contact by Kim Harrison- A portal opens and demonic beings come through. Renee is drafted by a secret government project to do research on these mysterious new beings that yields decidedly interesting results. A blend of scifi and fantasy, this first contact is not what anyone expected and may spell the salvation or downfall of the "neighbors" who are desperately hoping to find a way to save themselves from an ecologically ravaged world.
7. Witch King by Martha Wells- If Kai doesn't count, I'm arguing his companion Ziede does. They've both been around long enough and he's not a YA-style immortal, so I'm counting them as more mature MCs. Kai and Ziede have been imprisoned in one of the few places that could possibly contain them and were freed by expositors looking for power. What follows is a story of their past and present as they travel through a world with a rich magic system recovering from a successful revolt against the expositor's empire and their attempt to conquer the known world. But all is not at peace, as time has passed and the world has begun to forget the purpose of the Rising World Alliance. Kai and Ziede lived through it and the people that imprisoned them know they would oppose the rise of an empire or the return of the expositors. What follows is a race to discover the culprits and protect a fragile peace balanced against the backdrop of painful history. If you're familiar with murderbot, this isn't a humorous story, but rather dark and hopeful. Trigger warnings include genocide, body horror, etc.














