One type is drawn to and writes almost exclusively for a single age group, whether it's picture book readers, young adults or something in between. Many authors feel they are meant to write for one particular age, and some are just more comfortable with one particular kind of writing. It makes sense. The learning curve for different levels is steep. The style, length, voice, plotting, etc., are completely different from one age group to another.
The other type of author writes across all age groups or at least several of them. I fall into this category. I don't write board books or easy readers, but have put a lot of effort into picture books, chapter books, middle grade and YA. The only thing I've had published beside the picture book are stories in Cricket, which hits the middle grade reader, but I've written pretty extensively in all these age groups.
Lots of us in the latter group have noticed a trajectory in our writing. Our subject matter ages as our children do. We tend to write for the age group into which our kids fall at the moment. Maybe we're opportunistic, but we know those kids, their worlds, their concerns. At least when the kids are young, we also tend to read a lot of other books for that age group, so we know the field. Those are huge advantages.
I don't write exclusively YA now that Abbie is a teenager, and I wrote my first attempt at a YA novel well before Abbie was in high school. But I do feel more out of touch with toddlers than I once did and a lot more versed in the ways of texting and ACTs and homecoming dances.
That's all good, but it raises a question. When Abbie moves beyond what is generally considered the upper age for young people's books, am I finished? Do I start writing for adults, or do I move freely among the age groups because I've experienced them all?
I don't know, and mostly I don't think about it. (In general, I try not to think about Abbie reaching adulthood and leaving.) But I have noticed one trend that could counteract the problem of not having a child in the house. Some of my older colleagues are now writing books for the ages of their grandchildren...