Children with mental retardation tend to have trouble learning and adapting to their environment. At very young ages, including infancy, they may seem to be in their own world (e.g., not interacting with the world around them). For example, they may not respond to their own reflections. They develop many skills and reach many developmental milestones at a later age than other children. For example, they often sit up, crawl, or walk at an older age, or develop social skills (e.g., understanding social rules and the consequences of their actions), self-care skills (e.g., dressing and eating without assistance) and speech and language skills later.

Children with mental retardation may have trouble learning in school (e.g., difficulty solving problems or thinking logically). In general, the more severe the mental retardation, the sooner these delays are apparent. Mild mental retardation may not be noticed until the child reaches school-age. In most cases, children with mental retardation have a greater delay in language skills than in other areas of development.

Some behavioral problems may occur in children with mental retardation. These include eating disorders, such as pica (desire to eat non-food items) and rumination (bringing up already swallowed food to re-chew), and self-stimulating movements (e.g., head-banging). These problems are less common in mild retardation and more common in more severe forms.