One big sign of developmental delays in a child is the lack of any reading skills before starting school. Even before kindergarten, most children of normal intelligence know their letters, can count to 10 or even 20, and can read simple words like "cat" and "hot."
The delay may be retardation, or it may be more of a learning disability, like dyslexia. The earlier the child receives intervention however, the easier it is to overcome the deficit.
It's crucial to start life skills instruction as early as possible for a mentally retarded child. Their lives will be better for it.