Yeah, for languages, if your are 11, your are already too old.
http://en.wikipedia.org/wiki/Critical_Period_Hypothesis
http://en.wikipedia.org/wiki/Critical_period
@parents
There is a lot of research on bilingual children.
I read a bit about it.
My strongest impression over the subject is that every article seemed to connect the development of the child's language to how much the child liked the language.
I don't believe in the critical hypothesis. Lucky me, some scientists are on my side (not all of them, but some).
I believe children suffer as much for learning a language as we adults.
Learning a second language is a heavy task, specially for them at such a young age.
I remember that this regression stage occurs very often. But I don't remember how or when.
What I do remember is that eventually L1 will become dominant over L2.
This seems to be unavoidable. Also this occurs around the age of 6.
Also L2 does interfere with the learning L1.
You should expect that your children may have more trouble learning L1 than a so called "normal" child.
On the other hand, bilingual children seem to score better in divergent thinking tests. These tests are linked to creativity.
Your boy'll do great. Be confident for him and he'll succeed.