This study examined the effect of age, duration, and amount of exposure to English on the acquisition of its phonology by native Swahili children and adults who learn English as a foreign language. The study used purposive, stratified and snowball sampling techniques. The data were collected from Mlimani Primary School and the University of Dar es Salaam (Mwalimu Nyerere campus) by using two experiments, namely the English vowel perception experiment and the English vowel production experiment. The data were analysed quantitatively using the Statistical Package for Social Sciences (SPSS) software, in addition to being analysed qualitatively. The findings indicate that the adults were more accurate than the children in perceiving and producing English vowels. Specifically, they indicate that there is a significant, positive correlation between the respondents’ perception of the vowels and their production of the same, and that there is a significant correlation between the children and adults’ age, duration, and amount of exposure to the language and their ability to produce the vowels accurately. It was concluded that the native Swahili EFL learners’ production of English vowels is significantly influenced by their age, duration, and amount of exposure to English. And that their perception of the English vowels influences the production of the same. Further research on the effect of age and exposure to English on other aspects of English phonology such as stress and intonation are recommended.