Why? Im confused by your point here, why not trust parental judgement?
Because giving birth/adopting does not endow one with good judgment. It does not endow one with understanding of child psychology, or implant one with 30 gigabytes worth of research into the effects of viewing particular types of material.
As illustrated by all the parents who buy games for their children without looking at them, or reviews for them. They certainly don't play them through first to check the cut-scenes!
I can believe that responsible parents may think an 18 is suitable for their "mature 15 year-old", and accept that they know their individual child better. Eight year olds playing 18s? No.
I personally watched all manner of horror films (Poltergeist, Exorcist, Psycho, The Birds) from 8-9 onwards. My mother would have sworn blind on an internet forum that she was a responsible parent! That I was unaffected by my viewing.
I had terrible nightmares and used to get scared about ghouls at night. 