@Butterbean9
The earliest references I can find for emotional age being around 2/3 of the chronological age is as follows.
www.asd-forum.org.uk/forum/index.php?/topic/12901-emotional-age/
First, a book review from the Autism Society of Wisconsin:
"Asperger Syndrome and Difficult Moments: Practical Solutions for Tantrums, Rage, and Meltdowns by Brenda Smith Myles and Jack Southwick, Autism Asperger Publishing Company, 1999. Previously published as Asperger Syndrome and Rage: Practical Solutions for a Difficult Moment.
The book begins by giving an overview of the characteristics of Asperger Syndrome and how they impact behavior. I would like to quote one that I find very meaningful, from the section entitled "Maturity," "A rule of thumb: The child with neurological difficulties has an emotional maturity level that is significantly below his/her chronological age. Particularly between ages 9 and 19, children with Asperger Syndrome may appear to have the emotional maturity of someone two-thirds their age."
Second, from Asperger Syndrome Information:
"Individuals with Asperger Syndrome also have difficulty applying information and skills when placed in different situations, that is, they have great difficulty generalizing information. Their difficulties with social situations lead to difficulties understanding social cues such as frowns, smiles, boredom, and other ranges of emotions. These symptoms intensify between the ages of 9 and 19 when these children often appear to have the emotional maturity of someone two-thirds their age. (Myles & Southwick, 1999)"
And thirdly from Perner's 'The Big Picture of Autism' (about 4/5 the way down the page):"In part because of physiological limitations, we often have great difficulty picking up on what comes effortlessly to many. Many children, for example, readily discover different teachers? styles and expectations even though those are never actually verbalized. In contrast, people on the spectrum may lack the neural wiring to effectively pick up on the cues help others adjust. To make matters worse, those of us challenged by physiological limitations must devote more attention to compensating for sensory problems. This less attention available for maturation and growth. Some estimate that individuals on the spectrum may have maturity level comparable to that of a person with two thirds of his or her chronological age. Thus, there are some things that some of us learn with age?but just more slowly than others. There are other areas where maturation doesn?t seem to help as much. Many of us just don?t learn to tell when someone is angry or sad, let alone what to do about this."
More recently
www.additudemag.com/grow-up-already-why-it-takes-so-long-to-mature/
www.additudemag.com/real-age-adhd-emotional-maturity-executive-functioning/
For me it was lived experience as a parent, plus so many other parents in person or online groups for ASD/ADHD and other co-morbids mentioning the same topic of emotional immaturity sometimes with advanced intellectual abilities over and over again, that it seems to be far more prevalent than not in kids with neurodiversity.