The development of human language and phonological functions begins in infancy, and the first language activities begin when infants are 3-4 months old, when they are able to respond to familiar people and sounds around them and make different sounds with more than two syllables. 7-8 months old, infants can babble and make sounds and syllables that are closer to normal speech. 1-2 year olds can correctly pronounce b, p, m, h, n and other sounds. At the age of 2-3 years, they can also pronounce k, g, d, t, ng, f, and other sounds very well. These consonants and vowels such as a, o, e, i, and u are some of the more early developed sounds. Some of the more difficult consonants such as z, c, s, j, q, x, zh, ch, and sh vary widely, and children as early as 2-3 years of age and as late as 5 years of age or later can produce these sounds correctly. Human speech and language functions keep developing with age until adulthood. However, for phonological function of pronunciation, 2-5 years old is the most important stage in life.