How much of this is due to richer kids having more access to mental health resources? While I wasn't poor growing up, my parents didn't have the means to let me see mental health professionals and as a result I went undiagnosed for 21 years.
There's also different reactions to various illnesses across groups. In some subcultures, you might get a special, nicer treatment for having an illness, whereas in others you may viewed as broken and less worthy of respect.
The former would probably have some people seeking out a diagnoses for the illness, whereas the latter would have some people intentionally avoiding a diagnosis for the illness.
