Phonics is a method of teaching children to learn the relationships between the letters of the written language and the sounds of the spoken language. It is the basis for reading and writing independently. It is the connerstone in teaching English in the Montessori method.

The following two phenomena are very common in China. The key reason behind is that most people have not learned phonics.

1. Only knowing some spoken English:

A large number of kindergarten and lower elementary students can speak some basic English. But they can not read. They do not have the ability of self-learning and self-development.

2. Mute English:

When the students move up the grades, they force memorizing the pronounciations and the meanings of the words by sight. They start to understand written English. But they can not prounce new words. Also they can not speak and listen well. 

Sensitive period means a window of opportunity where experiences have greater impact on certain areas of brain development. Following are the sensitive periods for children to learn penmanship and reading:

Penmanship: 3.5-4.5 years old

Reading: 4.5-5.5 years old

Therefore, the optimal age to learn phonics is 3-6 years old.