|They don't make them like they used to, and that's probably a good thing.|
While it's true there are suggested word counts for various age groups (See Jennifer Laughran's excellent blog entry Wordcount Dracula) book length is determined more by publisher's economic needs than reader's ability to stick with a story. I am not saying publishers are greedy, just that they have to price their product low enough so it will sell. In an attempt to do this, many "books" have been subdivided into series. Look at the total word count for these favorite young adult series:
HUNGER GAMES - 3 Books - 301,582 words
TWILIGHT - 4 Books - 587,246 words
HARRY POTTER - 7 Books - 1,007,300 words
TWILGHT alone has approximately as many words as WAR AND PEACE. HARRY POTTER has over a million.
I don't think that's instant gratification. Young readers need to be given the respect they deserve.