Phonological Development (CLA)

Important: No two children develop at the same rate, and so no data charting the development of language and phonetics can be taken as gospel.

On average, spoken language begins to develop when children reach roughly the age of one. This is usually the point at which they utter their very first words, and is called the Holophrastic stage of child language acquisition (CLA). At first, words are not likely to sound exactly as the child intended, but often they still hold some sort of "phonetic resemblance" and are still used to convey a particular meaning consistently. Often a child's lexis will initially revolve around local topics, things they can see or touch, and their vocabulary will be primarily or entirely comprised of high-frequency lexis.
   
Because some sounds or phonemes are harder to produce than others, they develop differently. Vowel sounds are often mastered before consonants, with the average child having learnt and polished all the vowel sounds by two and a half, and only two thirds of the consonants. When consonants occur at the beginning of a word, children are often much more able to pick them up. They also tend to favour consonants when they are 'stops' (p, t, k, b, d, g), 'nasals' (m, n) or 'glides' (y, w), as they are easier to replicate and pronounce. The last consonants a child is likely to produce correctly are fricatives like 'v' or 'z'. The 'th' sounds also prove difficult.

It makes sense that those sounds which occur most commonly are learnt faster, as the child is more able to practise with them. One or two syllable words are most likely to be used by a child in the early stages of development, and 'a' is often the vowel first mastered.

Children often attempt to simplify sounds in order to make their replication easier:

Deletion - codas (end consonants) can by dropped by the clipping of words, syllables which are not stressed can by dropped (banana = nana), and consonant clusters can be reduced to make pronunciation easier (sleep = seep). Consonant clusters tend to be the thing children most struggle with, as they find it difficult to produce all the sounds needed easily and quickly, as their adult models do.   

Substitution - also known as the fis phenomenon, children substitute more difficult sounds with those they have already mastered. For example, 'there' can be changed to 'dere', 'that' to 'nat', 'thumb' to 'fumb'. The fis phenomenon helps to demonstrate that speech and language are independent of each other, as a child can understand the word fish and believe they have produced it correctly, when in actual fact the word they are producing is 'fis'. An example from The Linguist List: "Is that your fis?" "No, it's my fis!"


Bibliography(ish):

http://www.education.com/reference/article/stages-language-development-first-words/
http://quizlet.com/4967917/stages-of-child-language-acquisition-flash-cards/
http://www.kidshealth.org.nz/speech-sound-development
http://revisionworld.com/a2-level-level-revision/english-language/child-language-acquisition/phonological-development
http://linguistlist.org/ask-ling/lang-acq.cfm