[inimino]

Not so much about smarts but about exposure. Most of the kids who get into MIT for math didn't start doing proofs at MIT, but in maths olympiads and such at a much younger age. Still, even given the formidable qualifications of the MIT undergraduate CS class, they probably overshot at the time with SICP. (I also read SICP for the first time well into my programming career, and completely agree with your assessment.)