My aunt is currently on her way to finding representation for her YA issue fiction and she found her inspiration working in social work. Likewise, my cousin is an attorney and writing murder mysteries. My other cousin enjoys writing fantasy for the younger crowd and is a substitute teacher. Of course, this isn't out of left field. We all have interests outside of writing and these are reflected in our chosen genres. In my writing critique group, we call this "organic". You can smell an unnatural piece a mile away. It's forced, choppy and contrived. Sometimes when adults try to write YA, it ends up looking like an elderly person in a sideways hat throwing up a hang-loose sign.
It's taken me a long time to discover my niche and even now I'm coming to terms with the fact that I just can't write some stuff. The martial arts background helps with hand-to-hand combat scenes but I'll have to take a freaking class before I can really pound out a publishable epic battle scene. It's okay to pursue progress in weaker areas but for the most part, we writers need to stick with what we're good at. In my case, it's character development, neat settings and gadgets with magical energy sources. If I ever try to write an issue book, it'll end up looking like Dennis the Menace with an attitude problem because I've never been around adolescents who have substantial "issues" as it were. If I tackle mystery, you'll know who did it before I would because I'm usually the last person to figure it out in Scooby Doo even. As for writing fantasy for the kids, they'd all end up sounding like Stewie from Family Guy, and that's probably not such a good idea for the 10+ crowd.