IIRC constant abuse tends to 'reset' the brain to earlier points of development where there was no abuse as it attempts to find less painful behaviour patterns. This results in delayed development of certain areas of the brain; most notably the prefrontal cortex that is heavily involved with decision making and social behaviour but that isn't fully developed until one reaches ~25 years old so I don't know what you mean by "should be old enough to understand" because they clearly weren't physically capable of it.

Source is introductory psychology courses. One of my professors is a researcher in child development and worked a lot with kids like the person you're replying too. Treating them like "pieces of shit" just leads to more damage, so chill out.

It never burned down just ruined the microwave