The author of this post has written to clarify that she meant to say 'is it true that it is easier (and more profitable) to break in to the 8- to 12-year-old market?' rather than 12 to 15s.
And the short answer is: no. I reckon it's harder. Harder because there are so many different levels of reading and subject interest in that age group. Publishers are certainly looking for books for that age but they really can't say what they want - they just know if they like something or not.
I've found it easier to place new writers in YA, because the market's a little more defined by then - teens are either going to read or they're not, so there's not the same element of trying to find a story that will persuade a kid to read (almost like forcing them to take vitamins). By the time they're teens you can presume that they're reading a book (mainly) because they want to. That extra bit of licence means you're more likely to find really interesting, challenging stories and some really cool writing in YA books.
Fundamentally - and this is true for every writer - you have to write the story they way you want to write it. If you really think your characters are 15, don't make them 8 because you'll have to change everything about your story. By all means write a different story for the 8s - just don't try to remake an existing one. It's not the same as changing a chick lit age group from 35 to 25 ...