I disagree about LOTR*, it was written for an adult market. Whether young adults read it is a different matter - I did when I was about 13, so I suppose so - but it was designed to be read by adults, and at the time of its writing would have been considered old in its subject matter, particularly against the background of the world wars.
Narnia is, technically, a children's book.
Sometimes a good way to tell is to look at where they are displayed in book shops. The main display of LOTR will be with the adult books, although there may be some copies in YA as cross over stock. Similarly, Narnia will be primarily displayed with the children's books with some cross over to adult, particularly collector's items.
I think fantasy as a genre lends itself to YA better than many others, although horror has a good pedigree there too, thinking Darren Shan and the point horror tales. How much sci fi there is as YA, I'm not so sure, although I was certainly reading Heinlein as a teen, and Ender's Game would be YA, I think, too. Dune is another one that possibly crosses over, in much the same way as LOTR; written for adults but often read at that teen/late teen stage when a dedicated reader is wanting to be stretched a little more. (paul atreides is quite young, and Alia grows with the series from child to adult)
Possibly, what my long-winded, rambling, reply is saying is that I think there's a market being defined that was always there, and is being a little hijacked since the HP and Twilight success. Good teen readers have always found something to read that's relevant to them (gone with the wind, anyone, Scarlett's a teen just finding out she can manipulate men at the beginning, Lord of the Flies).
My worry is that if we write specifically for such a predefined market we miss what made some of the stuff mentioned so good; they were stories, told the way the writer wanted it to be (gone with.... is a good eg of that, one story, told by one person who just wanted to tell it, so is LOTR), written in a well rounded, challenging manner, that became accessible to any competent reader and could be enjoyed as such.
*but it's given me some hope, as I'm still scratching my head over mine and wondering if I want it to be defined as YA**, when I intended it for an adult market. Or rather, I intended to write the book I always wanted to, and didn't think about what age it was for.
** which means I'm talking myself out of a submission, and must stop this post now.