Infantilization

Infantilization is the prolonged treatment of one who has a mental capacity greater than that of a child as though they are a child. When used in reference to teenagers or adolescents, the term typically suggests that teenagers and their potential are underestimated in modern society, and/or that adolescents are often regarded as though they are younger than their actual age. 
Studies have shown that an individual, when infantilized, is overwhelmingly likely to feel disrespected. Such individuals may report a sense of transgression akin to dehumanization. 
There can be an overlap between the terms "infantilization" and "patronization", although infantilization derives more specifically from a sense of age group or hierarchical seniority on the part of those responsible for infantilization. The act of infantilizing others has been associated with narcissists.
Infantilization may also refer to a process when a child is being treated in a manner appropriate only for younger children.