Education is often seen in Europe as an important system to maintain national, cultural and linguistic unity. Prussia instituted primary school reforms expressly to teach a unified version of the national language, "Hochdeutsch." One significant reform was kindergarten, whose purpose was to have the children spend time in supervised activities in the national language, when the children were young enough that they could easily learn new language skills.
Since most modern schools copy the Prussian models, children start school at an age when their language skills remain plastic, and they find it easy to learn the national language. This was an intentional design on the part of the Prussians.
In the U.S. over the last twenty years, more than 70% of non-English-speaking school-age immigrants have arrived in the U.S. before they were 6 years old. At this age, they could have been taught English in school, and achieved a proficiency indistinguishable from a native speaker. In other countries, such as the Soviet Union, France, Spain, and Germany this approach has dramatically improved reading and math test scores for linguistic minorities.