May 23, 2011 · Immature Personality Disorder usually develops in teenagers, and is indicated by the absence of mature behavior. Although people with mature behavior know the difference between maturity and childishness, this disorder makes the patients act childishly all the time. [Page-Contents] An Overview of Immature Personality Disorder What is Immature Personality Disorder? Try to find as much meaning as possible in other relationships, so that your time spent with immature adults doesn't weigh you down as much. When you have to spend time with an immature adult, try to treat yourself to a relaxing evening if you stayed patient with them. You deserve the reward!