First of all I am aware that most of the books written by authors who are young adults or teenagers are very bad. However I think that people now don't even give a chance to younger authors because they automatically think: the author is young = the book will be crap. Older authors tend to look down upon younger ones but I've read books by people in their 40s and 50s that were BAD.
Update: I think people hate young authors because they rarely take them seriously, even when they deserve it. I've seen someone publishing a great book when he was 18 and when he asked the opinion of a published author she told him it was terrible and no one would publish it.