Every romance heroine doesn’t have to be in her 20s. Every amateur detective does not have to be a Girl Sleuth. I particularly enjoy novels in which the women 
protagonists are unapologetically older and use their wisdom and life experience to triumph over whatever adversity comes their way.
I recently re-read Killers of a Certain Age, a contemporary mystery novel by Deanna Raybourn. Billie, Helen, Natalie, and Mary Alice have worked together for decades as team of trained, highly-skilled assassins. Now, in their early sixties, they are invited to a swanky cruise on a luxury yacht to celebrate their retirement from a clandestine organization they refer to as The Museum. Fortunately, they have been trained to be cautious and observant wherever they are, and Billie recognizes a fellow assassin from The Museum aboard the yacht. A fellow assassin who has not contacted them. It soon becomes apparent that a peaceful retirement is not on the cards for this experienced team of women who must stay one step ahead of all attempts to silence them for good. This is a lively, energetic, suspenseful novel about teamwork, trust, and friendship. I adore this book and I will probably read it at least three more times.
Another, darker, more suspenseful novel, The Change by Kirsten Miller, includes elements of magical realism and is set in Long Island, New York. I very much enjoyed the three main characters, Harriett, Nessa, and Jo, and was pleased that they’re strong women past middle age who are discovering their true strength. The elements of magical realism — hot flashes that spark real fire! second sight! talking with dead people! — are beautifully integrated into the plot. The three friends discover a teen girl’s body near a hiking trail in a posh billionaire summer destination village. They use their unique gifts to discover what really happened, which leads to more bodies and more suspects and more dark secrets lurking behind the picture-perfect facade of a wealthy community. The mood of the novel is dark, propulsive, and suspenseful. Jo, Harriett, and Nessa learn to embrace their powers and their inner rage to work toward transforming their community and their own lives.
On a lighter note, Vera Wong’s Unsolicited Advice for Murders by Jesse Q. Sutanto, is an over-the-top, screwball comedy of a mystery novel with an older woman as the amateur sleuth. Vera Wong, in her sixties and widowed, doggedly opens her teahouse in San Francisco’s Chinatown every 
morning, even though she only has one customer per day. Vera lives above the shop, and one morning she arrives downstairs to find the dead body of a young man on the floor of the teahouse. She calls the police, but when they don’t behave like the CSI investigators she’s seen on TV, she decides to devote her time and talents to solving the case. She’s helped along by strangers who come to the teahouse to gawk at the murder scene. But are they really strangers or do they have some guilty association with the corpse? This is a heartwarming, funny story about finding connection in the most unexpected ways, even murder.
Other novels with older women as protagonists include:
Royal Holiday by Jasmine Guillory (contemporary romance)
The Benevolent Society of Ill-Mannered Ladies by Alison Goodman (historical mystery)
Iona Iverson’s Rules for Commuting by Clare Pooley (contemporary fiction)