I'm wondering what people consider to be truly adult science fiction then?
Adult-oriented is fairly easy to identify. The metric of "challenging at an adult level" sounds like a lot more of a moving target. Different people might find Children of Men "challenging" or not, depending on how that word is defined or whether or not it's viewed as a good thing. With sufficient snobbery-fu one could rule out all of screen SF as "challenging at an adult level" with the exception of The Discreet Charm of the Bourgeoisie or something.