Answer
Mertesacker was indeed 21 years old during the 2006 FIFA World Cup. Born on September 29, 1984, the German footballer was in the prime of his youth during the tournament, which took place from June 9 to July 9, 2006. This was Mertesacker's first World Cup appearance, and he made a significant impact on the German national team, contributing to their run to the semi-finals.