Why do I have a 9 year old brother that acts like a five year old?

There could be several reasons why your 9-year-old brother behaves like a 5-year-old. It might be due to developmental differences, where he may be maturing at a slower pace compared to his peers. Environmental factors, such as family dynamics or his social interactions, can also influence his behavior. Additionally, he might simply enjoy playful activities that are typically associated with younger children.