The practice that people classified as adults are superior to children in one or more ways simply because they are adults.
Many deprived children can develop serious depression from this and develop a feeling of hopelessness as well as psychotic problems.
Son: "I'm a human who has the ability to understand and evaluate, I can decide whether or not Pot is a harmful herb."
Father: "No you cant your 16, and anyway I own you you cant do that to my body."
Son: "You silly adultist."
Adultism is bad.