Research has indicated that bullying can start as early as the preschool years, particularly at the age of 3. Some kids are hardwired or are inclined to be aggressive compared to others. Children can also become aggressive if they are exposed to constant aggression as they tend to copy behaviours they observe from others. Media on television or online video channels also play a role in influencing childrens behaviour. Bullying can be experienced by children wherever they may be, including the home where they may have older siblings and/or parents who exhibit bullying behaviours.”]
Source: https://blog.malwarebytes.com/101/2016/11/anti-bullying-week-monkey-see-monkey-do/