It is important not to count on formal diagnosis as the only sign of a reading delay. Parents, as well as teachers, need to be aware of how there child is progressing and to act immediately if a problem is suspected. Early intervention is key. The earlier the student’s disabilities are identified, the earlier a specialist can target the behavior. Catching reading difficulty as soon as possible and initiating a remedial process can help to reverse the issues by working on weaknesses while building upon strengths.
Please see How Do I know if my child needs a reading tutor? for some early signs of trouble organized by primary grade level.