Infants have a unique ability to discriminate speech-sound (phonetic) differences, but over time they lose this skill for differentiating sounds in languages other than their native tongue. For example, 6 month old babies who were learning English were able to distinguish between similar-sounding Hindi consonants not found in English, but they lost this ability by 12 months of age. Since the 1980s it has been known that infants start focusing on their language’s consonants and vowels, sometimes to the exclusion of non-native sounds. More recently, researchers have increasingly focused on how infants handle whole words.
Recent research has shown that during infancy, babies learn not only individual speech sounds but also the auditory forms of words; that is, babies are not only aware of the pieces that make up a word, but they are aware of the entire word. These auditory forms of words allow children to increase their vocabulary and help them to eventually develop grammar. Although they may not know what the words mean, children as early as 8 months start learning the phonological (sound) forms of words and are able to recognize them—and just being familiar with the words helps increase the children’s vocabulary. Studies have shown that 18 month old children who are familiar with a word’s form are better at learning what it means and are also able to differentiate it from similar sounding words.
You'll be starting ENGA1 Language Development quite soon as part of your AQA A English Language AS (or will already have studied it as part of your ENA1 unit in AS last year), so keep an eye out for research into child language to help you understand the topic.
ENGA1 - Language Development