This paper focuses on generalizing different theories towards the age effect on the ultimate attainment of second language learners since it has long been a controversial topic in researchers’ mind. In this paper, it gives evidences on cases in favor of the Critical Period Hypothesis, which claimed the loss of language learning ability after puberty and presents counter-evidences on the successful acquisition of second language in adult learners. It has reached into a conclusion that consists the viewpoint drawn on previous analysis and confirmed the possibility in ultimate second language attainment for late learners.