Not saying it to be antagonistic, but I have always felt SF, as a general rule of thumb, is a tad more demanding of their targeted age group (in this modern age that is. Older SF had its own share of stinkers while they worked out the kinks), but this could be due mostly to hard SF being part of the discussion.
The way I see it, SF (medium-to-hard-boiled; the softer SF can be almost indistinguishable from fantasy at times) often generously pulls from derivative jargon, science references, and anchors cultural/historical/philosophical changes based on what humans have seen so far. As such, a reader will need greater baseline knowledge to understand and enjoy the work, more so than if he/she were reading Fantasy (unless it is a very grounded Fantasy where a reader's previous historical knowledge might enhance the experience). By no means am I saying "SF fans" are smarter than "Fantasy fans". One can rarely divide the issue so freely. Everyone reads both genres, and generally speaking, I believe fantasy works actually edge out SF in the literary aspect, but that's another topic I'm not willing to ignite--don't wanna be burnt at the stake, you know
).
IMO, all things considered equal, SF would require a little more effort on the reader's part because of its very own derivative nature. SF is also a child of the times. It is more current and anchored in current progress, which also makes it age way worse than fantasy. I believe that's the reason there are more highly-regarded Fantasy classics than SF classics, and not so much because of an actual difference in the level of storytelling/technical quality (or so I choose to believe
). A classic, by definition, must stand the test of time. SF rarely does, unless of a very advanced or speculative nature.
But, since we're analyzing the whole issue, I could also point out that maybe SF isn't more "adult", maybe SF writers miss-classify their work more than fantasy writers? Maybe something about SF makes it be incorrectly judged in terms of target demographic, or maybe the way it is marketed is at fault? I'll leave that there for someone actually knowledgeable on the subject to pick up
.
Just my 3.87 cents on this.