Not only does the development of personality occur during early childhood, but much of it is considered to be complete by early adulthood. Formative influences occur mainly in early childhood, and physiological factors that contribute to the development of personality are already in play. Having said that, the scholars at Stanford University’s Psychology Department have made a fairly convincing argument for the continued development of personality in early adulthood. [“Personality Development in Adulthood,” www.psych.stanford.edu/~lifespan/articles/Adult%20personality%20development%20(2001).pdf]. As the article from which the quotation included with the original question notes, “because personality draws so broadly from so many areas of psychology, including cognition, emotion, psychopathology, and motivation, consensus in the field over basic definitions is difficult to obtain.” In other words, there are so many variables that influence the development of personality, that anything is possible.
There is no question that personality can evolve through early adulthood. The natural process of maturation involves changes in how individuals respond to external stimuli and in how they process information. Development of personality certainly continues through the high school and even the college age years. Those experiences help to shape the people we become, especially for those who attend college distant from their homes and are living independently for the first time. Children begin to assert their independence early in childhood, but the practical manifestation of that independence may not become apparent for many more years because of the restraining and guiding influence of parents.
To extent to which personality continues to develop into adulthood, however, is at least partly influenced by the existence of physiological disorders to predetermine to a large extent an individual’s personality. Psychotic tendencies that manifest themselves early in life are likely precursors to personality traits that carry on throughout the individual’s life, barring pharmacological or therapeutic intervention that addresses that underlying condition.
In summary, then, there is much validity to the conclusions of the Stanford University researchers. One should guard, however, against a tendency to take those conclusions too far and neglect the more important influences on personality that occur during childhood.