It is a common belief that children, when compared to adult, acquaint themselves easier with a foreign language. This phenomenon has been described as the "critical period hypothesis" by Long and Lenneberg. Second language fluency can be subdivided in language comprehension and pronunciation. This paper argues that age of acquisition (AoA) has a negative effect on comprehensibility and pronunciation in second language (L2) acquisition. Only few early, and none of the late learners exhibited actual native-likeness across grammatical and phonological tasks. Age has a substantial effect on how bilinguals represent words. Either in spoken or sign language, adults who experience little or no accessible language in early human development dramatically alters their capacity to learn language throughout life. AoA has a significant effect of regular frequency words, with increasing AoA correlating with increased frequency effect for regular words and decreasing correct grammatical judgment at morpheme level. A possible interpretation of higher activation in cortical areas involved in language comprehension could imply that language acquired earlier in life (L1) is less effortful than L2 when acquired later. Cortical representation of language comprehension becomes more focussed within superior and middle temporal areas during late childhood. Moreover, AoA is linearly and negatively related to activation levels in anterior language regions and positively related to activation levels in posterior visual regions. For the prediction of a foreign accent, AoA is found to be the most influential predictor. Overall, grammatical and phonological intuition decrease with high AoA, with AoA ranging from 1-6 to be the most likely to perform native-like.