It is generally accepted that second languages are learned less well than first languages: most adult learners do not attain native-like proficiency, and the learning process requires time and effort.  Age is clearly a factor in second language acquisition, and the critical period hypothesis attempts to formalise this fact.

The critical period hypothesis posits maturational constraints on second language acquisition such that it is impossible to reach native-like levels of proficiency after a certain age, generally considered to be around puberty.

Research in this area (see Long) focuses on learning efficiency in younger and older learners, as well as ultimate attainment. 

Generativists seek to establish to what extent Universal Grammar is still available to older learners. 

Emergentists view poor second language performance as an inevitable consequence of cognitive development during first language acquisition:

An unfortunate drawback to the extreme efficiency of L1 processing, in particular to the developmental sharpening that it entails, is that adults are rendered 'disabled' second-language learners later in life.

Read more: Phonological transfer