Teenagers and twenty-somethings dominate in Northanger Abbey, which is largely about growing up and maturing. Catherine in particular progresses from immature behavior and attitudes towards a much more mature awareness. Youth is also closely linked to the theme of "Foolishness and Folly." Granted, folly isn't limited to the young in terms of age (see General Tilney). Nor are all young people prone to folly (see Eleanor). Youth and a subsequent lack of experience often lead to folly, though. Youth here is more of a potentially problematic state of mind than anything else. And growing up is a matter of mentally and emotionally maturing for many characters.
Though Catherine has learned a lot of lessons, in many ways she has not really matured or progressed by the end of the novel.
Catherine is much more mature at the end than she was at the beginning of the novel.