You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Switzerland

Context:
Education in Switzerland is very diverse because the constitution of Switzerland delegates the authority for the school system to the cantons. There are both public and private schools, including many private international schools. The minimum age for primary school is about six years in all cantons, but most cantons provide a free "children's school" starting at four or five years old. Primary school continues until grade four, five or six, depending on the school. Traditionally, the first foreign language in school was always one of the other national languages, although recently (2000) English was introduced first in a few cantons.

Question:
What is the minimum age for primary school in all cantons?

six