Second Language Acquisition

One of the concepts I found most interesting in these readings was the development of interlanguage.  I have taken many classes on second language acquisition but now I feel like I have a better understanding of how the interlanguage is formed.  In her book Introducing Second Language Acquisition , Muriel Saville-Troike explains in detail, the characteristics of interlanguage, which include systematic, dynamic, variable, and reduced system (both in both form and function).  This tells us that the IL is discovered by what the learner can produce and interpret.  I also found universal grammar to be an interesting topic because Chomsky tells us that language abilities exist in children and they only need to build on it to learn the language.  Is this innate resource still available to people who are acquiring additional languages beyond the age of early childhood?  If not, at what age do we lose this innate ability and why?