Granted, with the exception of the Harry Potter books, I don't read fiction intended for young adults.  But the qualifier "Young Adult," in my general experience, seems to apply most readily to a class of cheaply produced mass market products intended to take advantage of whatever fleeting trend has most aggressively taken control of that marketplace during any given quarter of the fiscal year.  Call it free association: when I think "Young Adult" (as opposed to "Children's Literature," which produces positive associations), I think Hollywood movie tie-ins, badly written adaptations rushed out to coincide with a theatrical release.  I think "Goosebumps" and any number of bad Buffy the Vampire Slayer-style, fantasy-high-school-drama rip-offs. 

Anyone familiar with Young Adult literature will no doubt find me abhorrently uninformed.  Looking at io9's own recent primer on current post-apocalyptic science fiction for young adults, however, I can't help but think that "Young Adult" really means "True to Formula."  If the books on io9's sampler list are any indication, the Young Adult genre is just another opportunity for market-savvy career writers to make an easy buck. 

The post-apocalypse is hot right now (we'll have more to say about this later) and, much like the hoard of secret-society penny dreadful thrillers that followed the success of The Da Vinci Code, the many YA post-apocalyptic series being written now are exploiting a profitable trend.  Or, taking it less cynically, it may simply be an example of a group of writers playing in the kiddie pool.  Young Adult fiction, even more than genre fiction in general, depends on formula to survive.  Authors interested in the post-apocalyptic genre might find YA a paid opportunity to write something that, at this point, would be too formulaic and cliched to fly as an adult novel.

Whatever the case, I'm inclined to think that any decline in mainstream science fiction and any subsequent rise in young adult science fiction are due more to a publishing industry too concerned with the financial bottom line to bother backing young writers with fresh ideas.  Every aspiring writer knows that those that get published got that way by arguing the market appeal of their work, not its artistic value.  Until the publishing industry becomes more interested in substance than massive sales, every genre will remain sparsely populated.