There are different stages of language acquisition, from babbling to forming complete grammatically correct sentences. Obviously the first stage is babbling which starts when the child is an infant as young as two weeks old. An infant can babble consonant sounds which are voiced and unvoiced; at this stage it is all the infants is able to do.
Around 6-10 months the child's babbling is more advanced and the child is more likely to babble sounds that he hears in his own linguistic community. Where as just a month or two younger he is babbling the same sounds as other infants not in his linguistic community. .
At about one year old a child is realizing that certain words belong to certain items. They are able to say their first words and connect them to an item, which is giving them a base component to phrase structure. .
Children around the age of one and a half grasped the concept of phrase structure and rules, but has not yet acquired the ability to use it. They can put together tow word utterances to tell you what they want, but cannot tell you what they know. .
Around two years old children have about 400 words in his vocabulary and their MLU is about 1.75. The child has still not moved on completely to deep structure but are well on their way. .
Children at the age of two and a half and three years old can understand deep structure and can use it. They tend to overgeneralize the past tense form "ed" like "goed", but have the concept down that you can make a sentences past tense. Children at this age have a vocabulary of 900 words and a MLU of 2.25. .
At three to three and a half years children can ask questions, they can use syntactic transformations, and they can understand transformational rules. They have a vocabulary of 1,200 words and a MLU of 2.75.
At the ages of 3and a half to five years old children have grown enormously in language. They jump from a vocabulary of 1,500 to 1,900 and a MLU of 3.