Not that I don't think they can handle it, just seemed that Ender's Game was pretty adult for a teen book. At least at the time. Now that The Hunger Games is touching similar topics, I guess Ender beating kids to death isn't as stand-out. Then again, Lord of the Flies handled it just fine, so maybe not so new a thing...
Speaker, while perfectly capable as a teen book, just felt more adult. The characters are now adults, adultry is a big part of what went on to set it up, the whole book's struggle felt more grown up. I read the whole series for the first time in the 5th or 6th grade, so it works fine, just my impression I guess. Nothing that FORCES an adult reading level, just better understood as an adult (or just not teenager) level I feel.