When does one detect any disability?

When a child is born the early detectable signs are- Delay birth cry, delay birth weight, if child is blue or yellow in appearance, if the child does not react to sound or light. 

As the child grows there may be delay in developmental milestones such as delay in sitting, standing, walking, talking, looking at things  or toilet control, these milestones are usually attained by typical children from 1 to 3 years of age. 

During school age from 3 to 15 years , the child may show less interest in studies, unable to read,  write or do maths.The child is usually lagging behind in comparison to typical children.