I think YA is the field that found me rather than vice versa. In the 1980s I wrote a novel with two seventeen-year-olds as protagonists. I just thought of it as "a novel" - I assumed (very naively) that it couldn't be a teen novel because it had sex and swearing in it. Then, around 1990, I had agent representation for this novel - as a YA novel. (It never sold, and I lost representation, but there you go.)
I have always read some YA - the last book I took out of the junior section of the library was Alan Garner's The Owl Service as I specifically wanted to read it. And I have read YAs off and on since then, mostly the ones that crossed over to adult readerships - Garner's Red Shift, Ursula Le Guin's Earthsea trilogy (as it was then - repackaged for adult readers), the Harry Potters, His Dark Materials, The Strange Case of the Dog in the Night-Time, Melvin Burgess's Doing It. In the last few years, when I've been concentrating on writing YA more, I've been reading it more systematically.
I'm in my mid-forties now, and I still find I'm writing much of the time about characters in their sixth-form (age 17/18) or University ages. I've even done this in short stories published in magazines aimed at adult readers. So I guess I have a strong impulse to write about characters of those ages.
Other reasons why I like YA is that you're not as restricted to one genre as you are in adult fiction. As I write short rather than long, I also like the fact that you can sell a novel of 45,000 or 50,000 words if that's what the story needs - you don't have to fill a minimum of 90,000 words as you do in most adult genres. I find that most adult novels are overlong these days. Also, I find the best YA has a freshness that's missing from a lot of adult fiction. Finally, most of the ideas I'm getting these days are for YA novels - I have enough to keep me going for five years at least.