The answer is: They had a rate of cognitive impairment several times higher than the children adopted less than six months of age.
Cognitive impairment refers to a condition when people experience some sort of trouble in remembering/learning past information.
The rate of cognitive impairment tend to be really influenced by the environment around us during our early years of development. Children who face adoption too late tend to experience a certain period when they were neglected and did not obtain proper nutrition. This is why the rate of convective impairment among them is several times higher than the children adopted less than six months of age.