The Maturity Model comes from John A. McKinnon, an American psychiatrist and founder of the Montana Academy.
Dr. McKinnon recognized that working with adolescence required more than just diagnosing and prescribing medication, that something else was contributing to difficulties at this age.

Works by Dr. McKinnon:

An Unchanged Mind: The Problem of Immaturity in Adolescence
To Change a Mind: Parenting to Promote Maturity in Teenagers