Exactly. Complexity of language applies to the under-11 set. After that, they've been taught everything they need to know to read anything ever written. Whether they've paid attention...well, not my problem.
It's amazing the vocabulary some kids pick up. I remember my son talking about "topiary" one day. I asked him where he'd picked up a word like that--cartoons, of course.
Or the time I showed a Goofy video to my first graders, and one of the scenes showed a chained-up elephant. One of the kids burst out, "Look at those huge shackles!"
If it's the right word, and you're writing for kids above third grade, you should go ahead and use it. If you're not sure, just craft the sentence so it gives context clues to help the kid understand the meaning.
For first and second-grade readers, it's different, of course, but that has more to do with the sequence of phonics instruction than with vocabulary. 6-year-olds with middle-class, educated parents know something like 100,000 words already. But they don't know how to sound out two and three-syllable words, or -tion endings, or words borrowed from foreign languages (like "foreign" and "language") that use unusual phonetic patterns.
