Unlike amnesia in adults, infantile amnesia is not a disorder but a normal condition:
a) it is one’s inability to recall events that occurred before the age of about 3 or 4 years.
b) some researchers explain this phenomenon as a consequence of the toddler brain’s immature

A) It is one’s inability to recall events that occurred before the age of about 3 or 4 years.

Explanation:

Before the age of 3 or 4 years, humans still haven't develop a strong capability to encode new information and storing it in long term memory. This makes us unable to fully remember the episodic events that might occurred to us before the age of 3.

Most people in the world actually experienced this type of amnesia. But many psychologists believed that the information that we received in that age still lies somewhere within our brain. But the data is missing many important parts so we are unable to retrieve it properly.