I can see both sides of the issue. On the one hand, if children keep their mother tongue in order to acquire basic literacy skills in their first language, it can help them to develop skills in the second language starting in grade 3. However, it is preferable for a true immersion program to start in grade 1. As a professional in second-language teaching, I feel that languages can be acquired at any age.
On March 29th, 2012. See this statement in context.