There have been studies that have many believing introversion and extroversion can be observed at a very, very early stage of life. I read it in a book about introversion and I'd love to give the specifics and such but the book is no longer here (Introvert Power was the book). Anyways, those I or E traits were claimed to be observable in infants. I believe we're hardwired at birth to be intro or extroverted. Beyond that, I think our types develop as we grow up. I also doubt genetics plays a huge role in things as my family is all sensor types.
Just my thoughts on the matter.