Learning a language by Immersion Immersion is the process by which children learn their native language, but it can also be used in educational settings. Apparently in some countries, where the official language is not the language that is actually spoken, immersion is used in schools to teach the official language. Some companies (like Rosetta Stone) also use the method of immersion in their language programs. 
From reading the Wikipedia article it seems that learning (a second) language by immersion is mostly used for children and does take quite a long  time.


Are there studies that compare the learning of a language by immersion and the learning by other approaches? 
Is immersion equally useful for adults and for children?
This is a part answer.  According to "Immersion Education for the Millennium:
What We Have Learned from 30 Years of
Research on Second Language Immersion", (Cummins, 1998), based on the Canadian example of French language immersion state:


  For immersion education to attain its maximum potential it must be integrated into an
  educational philosophy that goes beyond just the discipline of Applied Linguistics. Students
  must have opportunities to communicate powerfully in the target language if they are going
  to integrate their language and cognitive development with their growing personal identities.


I noticed this as well when I was a teacher in Japan (I am Australian), where (and this is an anecdotal observation) multilingual students seemed to pick up the language by immersion, rather than the classroom.  The same applied to me, in Japanese classes I did not do too well, but immersing myself into the surroundings, I was able to pick up the language (at least the functional parts) very quickly.

A great reference is "Integrating Language and Content in Immersion Classrooms:
Research Perspectives" (Swain, 1998), making an important point that the traditional way of teaching aspects of language, e.g. grammar in isolation, i.e. without a context is not a particularly effective means of language development.