I'm Confused By "Maturity" In Psychology And Psychiatry.

I'm a little confused by the concept of maturity in psychology and psychiatry. Recently online I read that in many ways those concepts are more social constructs than scientific or psychiatric concepts and diagnosis. I don't know because I don't have a degree in psychology.

I am also confused about my diagnosis and problems. My psychiatrist finally told me last year, I believe, that I do not have schizophrenia. Probably a personality disorder like schizotypal personality disorder or schizoaffective disorder. And my behavior does seem immature sometimes. At least to me and others around me. People have noticed all my life, even when I was a child. But in hindsight my behavior might have just been unusual or they may have been exaggerating. Ditto for how I have seen myself thru the years. I do have a theory surrounding, silly as it sounds, playing with dolls. People often say that is immature for an adult. But an adult who simply collects dolls is not behaving unusual for that age. But one who plays games of pretend with them might be.

What is maturity, and emotional maturity in psychology? How is it defined? What are its symptoms? What can it be a sign of? When is it clinical or a problem? And how does it relate to IQ?