Is there a critical period for language? When the sad story of Genie was broadc

Study of Genie supported the idea that there is an early critical period for learning language.
The likelihood of fully mastering a first language declines steadily after early childhood and falls off drastically at puberty.
A drawback to case studies is that information is often missing or hard to interpret.
Observers may have biases that influence which facts get noticed and which are ignored.
Memory of observers may be selective or inaccurate, making conclusions unreliable.
Most important, the person studied may be unrepresentative of the group that a researcher is interested in.
The case study method has only limited usefulness for deriving general principles of behavior.