I recently read Sue Townsend's Adrian Mole: The Prostrate Years, the latest book in her long-running series about a hapless intellectual from Leicester, England. I've been a fan of Adrian Mole since I was about twelve or thirteen, the same age that Adrian is in the first book in the series: The Secret Diary of Adrian Mole, age 13¾. Chronologically, Adrian is a couple of years older than me but we've had a lot of similar life and world experiences.
As a follower of series fiction, I get the sense that my experience is a rare one. Because of gaps in publishing, main characters usually age at a slower rate than their readers. Or series often end when the narrator comes of age, or is about 18 or so. At that point, the author has the option of shifting the focus of the series to another, younger character in an effort to keep it going. Sometimes this works very well, such as in Elinor M. Brent-Dyer's Chalet School series. Sue Townsend also did that very well in Adrian Mole although she had a different problem: a few of her key characters were elderly and passed away over the course of the series.
All of this led me to do a lot of what if planning when I started to write my series. My original plan was for four books that followed my narrator through high school. But then I started thinking about what Sarah would do in college, later in life and who she would marry. I ended up deciding that even I never got past writing the first book in the series, it was relevant to know what happened to her in five, ten and fifteen years because the choices that she made at fourteen would have some kind of impact on her life.
Sue Townsend said that she was finished writing about Adrian Mole after Adrian Mole and the Weapons of Mass Destruction. She cited excellent reasons – a little burnt out of the character and wanting to do other projects. But she's written two books about Adrian since then and I hope that The Prostrate Years isn't the end of the road for the series.