mental age isn't a thing.
i am not "mentally a child." i am mentally an adult, because i am in my early 20s. i do not care how childish i seem; that never gives anyone the right to say that i am mentally a child. i am an adult who doesn't want nor deserves to be compared to a child.
i do not care how many times i watch sesame street, i do not care that my reading level is low, i do not care that i need lots of things simplified to me, i do not care about all the things i do that make me seem childish. i still am an adult and deserve to be treated like one. i will never be "mentally a child."
this theory hurts people with intellectual disability and i am tired of seeing it.