The length of the average human arm is 25 inches. However, the length varies because 25 inches is the average for a young man with average height and health.
Average human arm length is as variable as average human height. It is widely accepted that there is a correlation between the two; if a person is taller, then they most likely have longer arms than a shorter person. This correlation is a little different in children, since they tend to have longer arms as they are growing in comparison to adults of the same height. For example, a 5-foot-tall child probably has slightly longer arms than a 5-foot-tall adult, assuming that the child hasn't yet reached maximum height.