An article about an older woman writing old women characters caught my attention today.
I'm not yet all that old (67) but there's lots of Alzheimer's in my family (and no long lives) and I can get a bit gloomy guessing how long I have to function adequately (and yes, as in the referenced article, I've acquired my last puppy, too — nothing but older dogs from now on). Despite that, I've started what is intended to be quite a long fantasy series (finishing the 3rd book now before starting to release the first 3). I'm keeping in mind the fate of long-running detective series where the hero gets too old too quickly, and beginning it with a prequel where my hero is 15 (then 20, by book 2).
It's a challenge. My explicit model (not for the story or setting, but for the slowly developing age of the hero and a long series) is C. J. Cherryh's “Foreigner” series: 1st book: 2004, 21st book: 2021, and still going. (Cherryh had already written dozens of books in SFF by then.) One of her major secondary characters is yet another old woman, soon to die at this point in the long story, and she is just as complex and functional (in her way) as anyone could want.
I find that when writing a standalone novel or a short series you can put just about any characters you want into the mix, but when planning a long series, you have to be more cautious about the “team” that constitutes the core — many of them are going to be with you for a long time, and you can't just kill them off, one per book, when you get tired of them, nor can you just accumulate the individual series book's new characters into the team promiscuously without that getting out of hand. You have to give the reader's view into your world a particular continuing focus to make things effective, and characters they can remember from book to book.
But an author's age/health does enter into series planning. I'm glad I had not yet begun releasing the first two books, because a health crisis interrupted everything for a year and a half (all better now), and makes me wary of the “book per year” minimal requirement. On the other hand, a shorter series (3 or 4 book, say) tends to have an overall series arc (like a fantasy quest) that you really need to complete, while a long series often ends without an overall arc — just the evergrowing weight of a team's long life with some sort of satisfying action at the end of each book. If the series ends prematurely with the author's life, less harm is done. So, I plan to keep plugging away on this new series and, with any luck, I'll live forever and start another long series or two afterwards.