Interesting question. I've found an answer in this wikipedia article. According to the article, it started in the decades before 1900 in some parts of Germany and since 1964 in whole Germany in every type of school in the 5th grade.
Since 2004, English classes are offered even in all elementary schools in Germany. In my opinion, this is good. Because children can learn a new language faster than teens and adults.
PS:
As a German, I'm cool with that. Because this avoids me to waste my life or a part of it to such a superficial woman. I already wrote: If we Germans give away our heart, it's hard for us to take it back. So we better don't give it to the wrong one.