The oblique effect refers to the phenomenon whereby visual performance is inferior in oblique orientations than in horizontal and vertical orientations. We compared the oblique effect among young adults, typically developing children (5- to 14-year-olds), and children with reading and writing difficulties (6- to 15-year-olds). Both groups performed a line-orientation matching task and a line-copying task. When the stimuli were oblique lines with either horizontal or vertical orientations, the oblique effect frequently occurred in both groups. When standard stimuli had orientations of 22.5°, 67.5°, 112.5°, and 157.5°, matching performance correlated with age in typically developing children but not in those with reading and writing difficulties. These results suggest that the perception of oblique lines gradually develops until the early teen years, and that this development is slower in children with reading and writing difficulties than in typically developing children. Orientation errors were larger in the line-copying task than in the line-orientation matching task in all three groups when oblique lines had orientations that were close to either horizontal or vertical. This is probably because the line-copying task requires finer finger control than the line-orientation matching task.