I'm a middle-aged (assuming I live to an be a very old man), tenured, geology professor and department chair and what do I sometimes do for relaxation? I read children's books. I admit it. Don't judge me. By the way, I know several other adults, some with PhDs, who also read children's books (and a few of them don't even have kids). Granted, most of the ones who admit to it are women, but I'm secure enough not to worry about it.
Now I'm not saying that's all I read, I read mostly non-fiction (I'm not a big reader of best-selling adult novels) but sometimes I just need something to relax with (I read every night in bed before going to sleep). I have two kids who are both voracious readers so that's why I'm familiar with many of the books written for tweens and teens (at any one time, we'll have between 30-40 books checked out of various local libraries sitting on our "library shelf").
Of course I've read the Harry Potter series (twice as a matter of fact, I reread the earlier ones when Deathly Hallows came out and then reread Deathly Hallows before seeing the movie). I've also read books by Rick Riordan (Percy Jackson series and others), Eoin Colfer (Artemis Fowl series and others), Lemony Snicket (A Series of Unfortunate Events), Spiderwick Chronicles (forget who the author is), and many others I simply can't recall at the moment. Most recently, I just finished The City of Ember (and have started the sequel The People of Sparks) since it was discussed at our local library's book club for tweens.
If I had any skill in writing fiction, I wouldn't write about adult themes - they bore me. I'd write about an alternative world where men carry swords, go on quests, fight evil creatures, and perhaps win the heart of a fair maiden or two. Much more interesting than my life of bills I can't afford, misbehaving kids, doctor's visits, grading labs, and chores around the house.