While the Silmarillion is definitely a different type of book — LoTR and The Hobbit are written in very similar styles (although The Hobbit is easier to read) and my kids were younger than 10 when they read them...I wouldn't call one an "adult" book and the other a "young adult" book. The writing in one is slightly more advanced than the writing in the other...as such, I think that you're overstating things.

I'm also not convinced that the Silmarillion is way above the target audience (young adults/adults). All of the books are fun to read, but you have to take a different approach for each one. If you're looking for a quick read--then Silmarillion isn't your book. However, if you want a complex story that builds an entire universe...then it's awesome--it's not a fast read, but that's why it's so good.