Words that we can recognise tend to first appear in typically developing infants anywhere between 12- and 18-months. After single words begin to appear, there will usually be a very gradual increase for 3 or 4 months, with the child having around 20 words in their vocabulary by the age of 18-months.
There is then a sudden burst with new words being produced at a staggering rate growing the child’s vocabulary from 20 words to something like 200 words by 21-months.
At a similar time, the child begins to string words together into small phrases. 2-word phrases may appear by 18-months – almost certainly by the time a child has acquired a vocabulary of 20 words. Regular 3- or 4-word utterences are typical from 24- to 27-months.
In developing language, infants pass through and progressively master, skills in four stages:
1) Rules about sounds (phonology)
2) Rules about meanings (semantics)
3) Rules about grammar (syntax)
4) Rules about social contexts and appropriateness (pragmatics)
By the age of 3-years old, a child will have a vocublary of 1,000+ words, and by school-age (5 years) children can typically use language similar to that of an adult. We will still see some logical errors – using words like ‘bestest’ or ‘worstest’ as the child grapples with some of the more complex language rules, but nontheless the child is able to understand and create complex sentences, and will adjust their speech to suit the age of the listener.