An important theme of True Notebooks is maturity. As the book primarily deals with young offenders, each of the students demonstrates a state of immaturity. Unfortunately, these different variants of immaturity are always relevant in their criminal history, and while none of the inmates of Central Juvenile Hall are reducible to their crimes, they are an unavoidable part of their personality and life history. The latter of these is the truth Mark realizes slowly, a realization hampered by the distance of his own life circumstance to theirs, and his desire to bond with them.

The former, however, reflects the possibility for change. This change is more relevant for youths, and made more tragic due to the length of their respective sentences. The legal system does not take into consideration the possibility of growth and change, but ironically, instills its own changes in individuals, through incarceration. The possibility of leniency is rarely extended to these inmates, given the grievous nature of these crimes. This is worsened by the environment of the prison, which further enforces these narrow ideas, and leaves little possibility for growth.

Critical to this process of maturation is the boys' realization that they can change, and that the attempts to "define" them must not succeed.