Changelings are in fact associated with autistic children.

Or, around 85% of cases can now be regarded as caused by a lack of neural pathway die-off. This occurs mostly in the 3rd trimester, then around 2-5 years old, and finally in adolescence. This was first theorised by Prof Eric Courchesne in the 2000s and 2010s.
I think you might be referring to language development in Asperger’s Syndrome, which has a typical trajectory until 2-3 years old, then declines compared to language development in neurotypical children. (Asperger’s was removed as a separate diagnostic category in the DSM V, but remained/ remains in the IDC guide at least until IDC 10.