Because they're idiots.
13 is the age or 18 1993
Originally from the book, the characters are supposed to be in their teens, around 14 to 20 or so. In the movie, some were portrayed by older actors.
at the very beginning of the book Dolores Haze (Lolita) is twelve and a half.
Aerrow, Finn, Piper, Junko and master Cyclonis are 14The others are unkown