After birth, the time of fastest growth in a person's life is:

The first twelve months. But a lot of crucial changes occur until the age of

five.

Explanation:

Infancy is the first year of life and it is the period of most rapid growth after birth. As a toddler, humans develop motor skills, ability to walk and talk, receive and return information. Early childhood is a time of tremendous growth across all areas of development when it comes to any individual. The newborn that’s dependent on his or her parents grows into a young person who can take care of his or her own body and is able interact effectively with others. The primary development of this stage is skill development.