Abstract: Age of acquisition is the age at which a word is first learnt. Age of acquisition effect is that words acquired earlier are processed faster than those acquired later. This paper summarized the methods of collecting of age of acquisition and the role of age of acquisition in adult’s lexical processing. It also described the relationship between age of acquisition effect and frequency effect, indicating that frequency is not merely age of acquisition effect in disguise. Finally, the paper introduced the mechanism of age of acquisition and emphasized the phonological completeness hypothesis

Key words: age of acquisition, age of acquisition effect, frequency effect, phonological completeness hypothesis