How old were you when you started learning English?
Do you think you would be more fluent and accurate in English if you had started earlier?
Did you learn English ‘naturally’ or in a language institution? Is one way better than the other or does it depend on age?
Children, adolescents and adults have neurological, cognitive and psychological differences in the acquisition of L2. Children seem to be intrinsically better learners, while adolescents and adults have cognitive skills and self-discipline which enable them to utilize a larger volume of comprehensible input within the same exposure time period. On the other hand, older learners may be more greatly affected by the other factors in L2 such as motivation, anxiety and identity. The difference in the acquisition process demands different approaches to instructing learners of different age groups.
Age of acquisition: What does research say? 
Age is easier to define and measure than personality, aptitude, or motivation. Nevertheless, the relationship between a learner’s age and his or her potential for success in second language acquisition is under debate.
Children from immigrant families eventually speak the language of their new community with native-like fluency, but the same does not happen to their parents. One explanation is that, as in first language acquisition, there is a critical period for second language acquisition.
Younger is better?
The Critical Period Hypothesis suggests that there is a time in human development (before puberty) when the brain is predisposed to success in language learning. Developmental changes in the brain, it is argued, affect the nature of language acquisition.
It is difficult to compare children and adults as second language learners. In addition to the possible biological differences suggested by the Critical Period Hypothesis, the conditions for language learning are often very different.
Younger learners:
- Younger learners in informal language learning environments usually have more time to devote to learning language.
- They often have more opportunities to hear and use the language in environments where they do not experience strong pressure to speak fluently and accurately from the very beginning.
- Their early imperfect efforts are often praised or, at least, accepted.
Older learners:
- They are often in situations which demand much more complex language and the expression of much more complicated ideas.
- They often get embarrassed by their lack of mastery of the language and they may develop a sense of inadequacy after experiences of frustration in trying to say exactly what they mean.
Critique of the Critical Period Hypothesis
The Critical Period Hypothesis has been challenged in recent years from several different points of view:
- At least in the early stages of second language development, older learners are more efficient than younger learners.
- Learners who began learning a second language at the primary school level did not fare better in the long run than those who began in early adolescence.
- There are countless anecdotes about older learners (adolescents and adults) who have reached high levels of proficiency in a second language.
Does this mean that there is no critical period for second language acquisition?
Patkowski (1980) found that age of acquisition is a very important factor in setting limits on the development of native-like mastery of a second language and that this limitation does not apply only to accent. These results gave added support to the Critical Period Hypothesis for second language acquisition.
Experience and research have shown that native-like mastery of the spoken language is difficult to attain by older learners. Nevertheless, there is some research which suggests that older learners may have an advantage, at least in the early stages of second language learning.
In the Snow and Hoefnagel-Hohle study (1982), it was found that adolescents and adults learned faster than children in the first few months of exposure to L2. By the end of the year, the children were catching up or had surpassed the adults on several measures. Nevertheless, it was the adolescents who retained the highest levels of performance overall.
Snow and Hoefnagel-Hohle concluded that their results provide evidence that there is no critical period for language acquisition. However, we should also consider that some of the tasks in the study (for example, sentence judgement or translation) were too hard for young learners.
Adults and adolescents may learn faster in the early stages of second language development (especially if they are learning a language which is similar to their first language) but young children eventually catch up and even surpass them if they are surrounded by the language on a daily basis.
Adults and adolescents can also make considerable and rapid progress towards mastery of a second language in contexts where they can make use of the language on a daily basis in social, personal, professional, or academic interaction.
The role of the critical period in second language acquisition is still much debated. Some researchers conclude that there are maturational constraints on language acquisition, but there are also others who consider that the age factor cannot be separated from factors such as motivation, social identity, and the conditions for learning. They argue that older learners may well speak with an accent because they want to continue being identified with their first language cultural group, and adults rarely get access to the same quantity and quality of language input that children receive in play settings.
Many people conclude on the basis of studies that it is better to begin second language instruction as early as possible if we seek the highest possible level of second language skills, the level at which a second language speaker is indistinguishable from a native speaker. But achieving a native-like mastery of the second language is not a goal for all second language learning, in all contexts.
What are the implications for educational settings? 
In the chart below you will find a summary of the most important issues to consider when deciding on the most appropriate age to start learning an L2 in educational settings. These ideas apply to both formal and informal educational contexts (i.e.: school education and private L2 classes).
Conclusions
There is strong evidence of the existence of a Critical Period for L2 demonstrating the value of early exposure to the second language.
However, the initial point of learning cannot play the same role in a naturalistic and in a classroom context; the earlier may be the better but provided that it is associated with enough significant exposure, other important conditions include that exposure to young learners should be intensively distributed giving learners opportunities to participate in a variety of L2 social contexts.
In the end, more important than the starting age of acquisition is the amount and frequency of exposure to the L2.
References
http://carla.umn.edu/learnerlanguage/ind_diff.html
Lightbown, P; Spada, N. (2001) Factors affecting Second Language Learning in English Language Teaching in its Social Context (pp. 28-43)
Muñoz, C. (2010). On how age affects foreign language learning. Advances in Research on Language Acquisition and Teaching: Selected Papers. Gala
Qin Zhao, A; Morgan C. (n.d.). Consideration of Age in L2 Attainment – Children, Adolescents and Adults. Asian EFL Journal