In The age factor in second language acquisition: a critical look at the Critical Period Hypothesis, Singleton and Lengyel (1995) make an argument against the "Critical Period" Hypothesis proposed by Wilder Penfield. In Penfield's hypothesis, the first years of life are the most optimal for first language acquisition and that by the time of puberty, neuroplasticity is no longer viable for the acquisition of new grammatical or syntactical systems. Therefore, all learning (language learning) should occur at the earliest stages of development for it to be mastered at all.
The criticism to this hypothesis comes from the fact that a) there is not enough scientific evidence that supports this claim, b) that other factors and variables come into play in the process of any kind of learning, and c) that the hypothesis leaves out a number of theories of learning and motivation which clearly point to the ability of different age groups to learn second languages properly, if the right elements are present at the time.
These "right elements" seem to be the one constant throughout all the theories of L2 learning: at the proper developmental level, using proper feedback and motivation, and at consistent and accurate exposure to the L2, any person of any age level can learn a second language. Neuroplasticity is a principle that applies to certain types of learning and, as the name implies, has little to do with age, but with constant practice and usage.
The one thing that is true, according to Krashen (1987), is that as long as
the early stages of syntactic and morphological development through time and exposure are held constant
older children, and even adults acquire language at an appropriate and accurate range. However, it is a fact that when a language is learned at a younger age and keeps being used, the language usage will become stronger than those who learned the language at a latter age. This, however, does not have much to do with age, but with the experience using the language. In not so many words, the more you use it, the better you get at it, regardless of what age you may be.