They have not even developed the understanding to concentrate on
a specific thing
for more than
a couple of minutes. My son was
a wonderful, patient, and could concentrate till he was about 5 years old. That is when we first saw some signs, and he went to preschool and had no problems. It was when he started Kindergarten, when he actually was diagnosed and the problems were definitely there then.
My cousin has a little boy that was diagnosed with speech delay when he was 2.