More 100+ year old children. I find this theme somewhat disturbing. With vampires, at least, I've seen it somewhere besides German children books; it seems high schools are overrun with them. Perhaps their brain ceases to develop when they become undead, and so they are forever stuck with thoughts and opinions they had before they died. That doesn't explain why the would want to do childlike or teenagerly things, nor how they keep up with societal changes, so in most cases it seems like it's nothing but a convenient plot device. Although Rüdiger and Anna aren't old fashioned, it does seem as if they have not matured a whole lot. Assuming the way you act is determined entirely by biological age, that might actually make sense.