Adults do come to the afterlife. The conditions for people to enter that world are...
They must have led a horrible life and died.
Happened while they are still being oder in the age of education. (So, going to school)
Age does not matter.
Like Chaa, the one in charge of the Guild. He's actually an adult, but his time of death was around the time when he was going to highschool. (read the manga)
I'm pretty sure where Angel – Jäger der Finsternis beats takes place is only a part of the after life. To be their Du have to be a high school student who had an unfufilling life. I'm sure adults go somewhere else if they had an unfulfilling life.