Research shows children can start acting mean in kindergarten, or even worse, before then.
The Wall Street Journal reports researchers observed children using the threat of withholding friendship as a "tactical weapon."
Laurel Klaassen, an elementary school counselor in Iowa, told the Journal: "They're already thinking at that age about being popular, being the queen of the classroom, or the queen of the playground and vying for that position."
This isn't the first time research has shown young children engage in bullying.
Education.com says, "studies conducted in different countries have demonstrated that bullying occurs at approximately the same rate in kindergarten as in elementary school."
The research used the phrase "relational aggression" to describe the type of bullying observed.
According to The Ophelia Project, a national nonprofit organization that focuses on reducing relational aggression, the term means "harming others through purposeful manipulation and damage of their peer relationships."
In an article published by the American Psychological Association, psychology professor Dr. Jamie Ostrov said relational aggression can occur just as frequently as physical aggression - it's just harder to detect.
Through research, he discovered some popular examples of relational aggression among 5-year-old children were "You can't play with us" and "You can't come to my birthday party."
According to The Wall Street Journal, research has found about 50 percent of children in 5th through 12th grade experience relational aggression on a monthly basis, and seven percent say they experience it on a daily or weekly basis.
Because these issues are emerging at such a young age, ABC says schools are beginning to implement programs to help teach kindness.
"Some schools are launching special programs to teach empathy, asking kids to put themselves in other kids' shoes or think about how it feels to be left out. I know in my house we have that conversation almost daily."
Although it's not clear why some children are prone to being more relationally aggressive than others, Ostrov says these behaviors can be learned by observing older siblings and parents.