As a smart teen, I love this book. It's one of my favorites. It's loads easier than the Iliad. (I read the Odyssey in four days compared to where it took me a month to read the Iliad) It's a great way to get into epic poetry. I would reccomend having some myth knowledge or having some sort of myth dictionary while reading it. (Try Mythica for a myth encyclopedia)
I would say gifted kids could read it at 12. I read it at 13 for the first time. Reassure them that it's okay if they don't get everything the book has to offer. There was many details you'll miss. After reading it three times now, I still find new details every time!