As a junior, Ehrhoff competed for Germany's under-18 team at the inaugural 1999 IIHF World U18 Championships, hosted in Füssen and Kaufbeuren, Germany. He returned the following year for the 2000 IIHF World U18 Championships in Switzerland, where Germany finished in seventh place. Ehrhoff then debuted at the under-20 level in Division I of the 2001 World Junior Championships. Hosting the tournament in the cities of Füssen and Landsberg, Germany missed a promotion to the main tournament the following year by finishing in second place behind France. Ehrhoff made his second appearance at the Division I level for the 2002 World Junior Championships in Austria, leading all tournament defencemen with three goals and 10 points in five games. He helped Germany earn a promotion to the main tournament for the following year, defeating Austria 7–1 in the gold medal game. In both years competing in the tournament, Ehrhoff earned Top Defenceman honours.