Why does neuroplasticity decrease in adults?
Another reason for reduced plasticity in adults is that learning something different in the presence of an existing knowledge structure is more difficult than learning from a "blank slate". In a sense, you get interference from the known language (for example). One person who has developed this argument computationally is Jay McClelland in the context of native Japanese speakers learning the English /r/-/l/ distinction (e.g., McCandliss et al., 2002), though I think the principle may apply more generally. 

The idea is that "reduced plasticity" is just a consequence of interference from the established knowledge rather than a fundamental change in plasticity. Part of the argument is that you only see reduced plasticity when there is a conflict between the (for example) first language and second language; aspects of the second language that match the first language are learned very quickly.