Question: When does child personality first appear?
Answer: There are hints of child personality from very early on in life. For instance, some infants crave routines while others prefer greater flexibility. Psychologists call these early clues "temperament". Temperament is innate and has been characterized in various ways by different researchers.
To most fully understand a child's "personality", however, we must consider more than just temperament. Personality is the sum total of an individual's emotional, attitudinal and behavioral responses. According to psychologist Dan McAdams, an individual's personality can be characterized by observing their specific personality traits, and that these traits don't appear in a clear and consistent manner until the tween years. Therefore, child personality emerges in the truest sense only as adolescence approaches.
Like temperament, personality traits have been characterized in various ways by different researchers. Perhaps one of the most prominent personality theories focuses on five key traits (called the "Big 5" by psychologists): conscientiousness, agreeableness, openness to experience, neuroticism and extraversion. McAdams notes that these five traits first crystallize during the tween years. In particular, researchers begin to find differences from one child to the next on each of the Big Five traits during the tween years. They also find general trends in levels of the Big 5 occurring for all tweens. For instance, conscientiousness tends to be on the rise during the tween years. The combination of readily observable individual differences coupled with overall general trends indicates that traits - and hence "child personality" - most truly emerge during preadolescence.
McAdams, Dan, & Olson, Bradley. Personality Development: Continuity and Change Over the Life Course. Annual Review of Psychology. 2010. 61: 517-542.