It seems like a silly question - but I've seen authors who didn't realize they would be writing a series for twenty years regret choices they'd made about age. If you start a character in her late forties - which is still, in my book, reasonably young - and you age the character in real time, i.e., you put out a book a year, twenty years later, your character is approaching seventy - which is a little old to be trading punches or climbing walls.
Daniel Silva, who writes the Gabriel Allon character in his espionage thriller, tied his character to a real historical event - the justice exacted by Israel on the killers of Israeli athletes in Munich in 1972. His character is now in his late sixties. It's still a good series, but Gabriel is getting a little long in the tooth to be personally wrestling with terrorists. Not that it stops him - but at some point, it will become questionable.
On the other hand, Miss Marple, in Agatha Christie's books, started out as a little old lady and never seemed to change. Of course, her activities were knitting, figuring out clues, and being adorable, activities that can be done as easily at an older age as at a younger age.
Sue Grafton and Sara Paretsky, both writing women detectives who were more likely to get shot or shoot back than Miss Marple, had different approaches. Sara Paretsky initially aged VI Warshawsky in real time. Books are published a year apart, and V.I. aged that year in between. However, in 2010, Paretsky changed V.I.'s birth year from 1950 to 1957. Kinsey Millhone in Sue Grafton's book did not age in real time. The events in Sue Grafton's books, although also published a year apart, took place almost immediately after the events of the previous book. But that made for a little caginess about in what year her book were taking place.
So what have I decided? I don't know if I'll be writing the Kolya Petrov series for twenty years - but maybe. I really like him, Alex, and the supporting cast. So I'd like to leave my options open. This means that I'd like him to do some aging, but not so much that I can't send him off on dangerous mission.
I've made two decisions. I started Kolya and Alex at approximately age 34 or so, which is damn young, and gives me a little flexibility. And 34, while young, is still old enough that both of them have had sufficient time to gain a reasonable level of experience and accomplishment in their professions of secret agent and attorney, respectively. But I'm also making the decision that the time in the books will not exactly correspond to real time. The events of Nerve Attack are close to real time - they happen eight months after the events in Trojan Horse, and the book will be published almost a year after Trojan Horse. The events of the book I'm working on now are four months after Nerve Attack, and the book will be published in the summer of 2022 (assuming I finish it). So in two years, Kolya and Alex will have aged one year. Which means that if both I and the series last twenty years, Kolya'll be somewhere in his forties, still young enough to be that active character that I'm writing, but not so old that I would feel guilty at forcing him to hang off the side of a cliff. After all, the series is in something of an alternative world - the President and all the politicians are fictional (although Bernie Sanders exists in this world) and there is no Covid - so why not bend time a little?
So what do you think? If you're a writer of a series - what decisions have you made about your character's age? If you're a reader - what age do you prefer for the protagonist - and do you have any thoughts about whether they should age in real time?