Im assuming its the assumption that the special fx are so dated that young people simply won't be scared but im not sure I always agree sometimes the content/dialogue is still geared towards an older audience. and example is JAWS, its a PG now on Netflix
the head bit even by itself is scaryish, and suspense and a child being eaten! And Indiana Jones apparently an PG
some adult themes there too. Is it just me? I'm sure there's more.