Compared to young children, adults have a much harder time learning a new language. Researchers studying this phenomenon are mainly addressing which key issue in development?

Critical versus Sensitive Periods

Explanation:

In the critical period of a child's development the if certain things are not learnt it becomes almost impossible to learn them at a later stage in life.

According to certain scientists it is believed that if things are not learnt at a particular stage then it becomes difficult but possible to learn.

Here, the learning of a language is being studied. It is true that learning a language when you are a child is easy but it is difficult to learn a language after a certain.

Hence, the key issue that is being highlighted here is Critical versus Sensitive Periods.