Sie or Du? The answer is similar to "Sir".

Adult to child = adult uses Du while child uses Sie.
Adult to adult = use Sie until ....
student to teacher = Sie
teacher to student = emphasize maturity = Sie,
talking to little kids or talking "down" to kids = Du.

Sometime during Sr High, kids start thinking of themselves as adults (or at least want to be taken more seriously). This means calling them Sir or Mr. Smith, etc.

While Americans tend to be very informal, we Germans still play this game. Germans are much more formal, hence the emphasis should be on Sie.

GERMANS STILL FACE SOCIO-LINGUISTIC DU-LEMMA

Non-native speakers of German take heart: according to a University of Bonn linguist, the Germans themselves are still occasionally flummoxed over the Du/Sie distinction. The choice between the informal and formal variants of "you," historian of language Werner Besch concludes in his new book Duzen, Siezen, Titulieren (roughly "Thou, You and Titles"), remains as problematic today as it was thirty years ago. In the heady days of the late 1960s, as Besch recounts, the student movement championed the use of Du as a step toward greater egalitarianism. Although Du has supplanted Sie as the most common form of address among university students, its hold throughout academia has weakened. Even many veterans of the student movement who pursued teaching careers, he reports, now find it embarrassing and awkward to use Du with students who are not only much younger than themselves but whose work they will eventually be called upon to grade.

Besch's book, the Frankfurter Rundschau reports, carries numerous examples of the Du/Sie dilemma from outside the academic world as well. In 1976, for instance, a vendor in Nuremberg's produce market found herself saddled with a hefty fine after using Du to a senior police official; in her native village, she argued in her own defense, everybody used Du with one another as a matter of course. And if it is de rigueur to use Sie with police officers and all other secular authorities, native and non-native speakers alike might legitimately wonder, why is it common to address God as Du in prayer? English-speakers, count your blessings.