Ours were struggling after 6 months - the breakthrough came at around a year and then the next 12 months it was leaps and bounds. When the kids started correcting their mum's German we knew we were on a winner.

The point about kids learning langauges at different rates and in different ways is valid - the head of ICSZ gave us a summary of some research from a conference he'd attended just before wqe placed our 14 year old there. I haven't got all the facts or references but the overview is:

There is a part of the brain responsible for the mother tongue and another part for acquired language. Although there is no abosulute fixed age it appears the part that is responsible for mother tongue becomes "fixed" at around 13 years. Before that it is possible to speak a language like it is the mother tongue, after that it becomes progessively more difficult. Languages learned after 13 are acquired languages and although you can become very good it will never be as good as the mother tongue.

In some children that part of the brain that looks after mother tongue is more flexible than others so some children may find swapping language difficult at an earlier age.

The other very big point is that children who have a parent at home who can speak the new langauge are much faster at developing in the new one - this is true across the age groups and I guess it's pretty obvious.