I cannot accept that excuse. That's just talking down to your readers if you write like that. It's insulting and I don't think that's what Scott was doing.
I reference a lot in my arguments, but in Margaret Peterson Haddix's Shadow Children series, she managed to make a group of people who killed children as part of their job description just as relatable as the main characters. She showed that they weren't evil without removing the stigma that they were still the enemy and were to be brought down.
That series is marked as being for ages 8-12, while Leviathan is marked for ages 12 and up. By that logic pre-teens and teens have gotten dumber since 1998 (Original publishing date of Among the Hidden).