Critical periods to the extent that they exist anywhere in development are more the exception than the rule. Still there are some interesting critical period phenomena in human devel. Any later and there will always be lack of binocularly driven cells necessary to binocular vision. New research comparing foreign language skills as a function of age of arrival in foreign country. Still, the bottom line is that the concept of critical periods that developed from embryological studies has limited applicability to behavioral, emotional, linguistic, intellectual, and social development in humans and other animals that manifest these.
Critical periods are no threat to the general rule of plasticity.
Indelible stamp made once during development.