Of course, I also write minor characters of all ages -- children, middle-aged, elderly. And they're all interesting to write. And, of course, MANY adult writers are penning young adult novels, where the main character's age is significantly younger. I wonder if, in those cases, those authors are using a niece/nephew or child as a guide (observing that generation close-up, like research).
But for me, for my core characters, at least right now, I like to feel their age, embody their age.
I wonder if that'll change, over time. If, when I'm sixty-something, I'll start to write fifty or sixty-something-aged main characters. Rosamunde Pilcher, now in her eighties, has seemed to write progressively older main characters as she's aged. And I wonder if it's because she can relate to them more--their time of life, their circumstances, etc.
What ages do you generally gravitate toward (depending on the type of fiction you write)?