Adam did not learn to read until he was an adolescent. When he did learn to read, it was much harder for him than it was for his peers. If Adam had learned to read during the early and middle childhood years, he would have acquired literacy skills much more efficiently. Adam's difficulty learning to read as an adolescent demonstrates the concept of a(n) " period."

Sensitive period.

Explanation:

From the question we can see that Adam started learning to read when he was already an adolescent and during this time of adolescence he has problem or difficulty at learning to read than his peers that have learnt how to read before the reach the adolescence age. This concept is known as SENSITIVE period. It is the period in which children are able to understand things. The sensitive period is from birth period to the age of six. The sensitive period is one of the periods for human child psychological development.