Youth. It seems to get all the glory and much of the oxygen when it comes to fiction and likely in reality as well. So being the rebel that I am, my fiction has always been centered around characters with a few road miles on them. My main characters, Cale and Lane, as well as Dana and Myra are all well seasoned. Cale and Lane have "aged out" of their program. Some of River and Ranch and nearly all of New Grass Growing is looking at what two men do in their retirement after a compressed highly charged career of kinetics and wet work for Uncle Sam.
It is interesting to read through your favorite characters with an eye on clues commenting on their age as well as on mentions of characters that are "older". Harlan Coben comes to mind first. He keeps his main character, Myron Bolitar, in touch with his parents. Myron from time to time goes back to his childhood house and roams his old neighborhood. I think this is a great way to keep a sense of intriguing reality woven through fiction. Put the characters in real time with characters from their past and their family. Not everyone is an orphan like Mitch Rapp (Vince Flyn) or Scot Harvath (Brad Thor). Hope I correctly spelled their names. Loved the books with those recurring characters, but the pace is breathtaking and in the present. Not a whole lot of "what came before". To be fair, both authors make an occasional reference to age. Vince Flyn also included a minor character, Stan, a nasty old relict from the Cold War, a chain smoking, dirty fighting, questionably ethical character as part of the Mitch Rapp ecosystem.
Anyway, my fiction is an homage to lives well lived. Sabé and Cassidy are high school kids and Peter and Perrin are elementary aged kids, but most of the characters are appropriate ages for parents and grandparents, with road miles to boot. River and Ranch and New Grass Growing are about late found love, raising teen agers, talking with families, and living a family-centric life in the flyover country of remote Idaho.
Along with a few chills and spills along the way of course.