Is there a 'critical age' when children shouldn't be moved to a school with a different language pattern in the curriculum? 

A critical age is when an aspect of development can only occur at one (usually very early) time in a person's life. In the 1960s, a debate existed about the plasticity of the brain. One viewpoint suggested that there was a particular critical period in young children for language to develop easily. Research has since suggested that there are no critical periods within a child's language development. Advances in language are possible later in life if they have not occurred earlier. A second language can be successfully acquired from birth or in retirement years.

However, there are often advantageous periods. Acquiring a second language very early in life has advantages for pronunciation. Developing a second language in the primary school is advantageous, giving an early foundation and many more years ahead for that language to mature. In the early years of schooling, a second language is acquired rather than learnt. Such advantageous periods occur when there is a higher probability of language acquisition due to circumstances, time available, teaching resources and motivation.

The question points to an important decision for parents who move to a different geographical area. Children vary considerably in their ability to adapt to a new language environment, and their attitude and motivation to picking up a new language quickly. The quality and quantity of practice in the new language available in the school and outside school also varies. However, as a rough guide, and with many children being exceptions to this guide, here is a suggestion. Up to the age of seven, there seems little problem with children quickly adapting to the new language environment of a school. The language used in the classroom in the infant years tends to be relatively simple and straightforward, and fairly quickly acquired by children. The infant classroom involves project work, plenty of actions and role playing, plenty of interaction with other children that enables children to acquire a new language relatively quickly.