Sergio Agüero joined the youth system of Independiente at the age of nine. He started playing for the side, becoming the youngest player to debut in the first division at 15 years and 35 days (on 5 July 2003 against Club Atlético San Lorenzo de Almagro), when Independiente's coach, Oscar Ruggeri sent Agüero onto the field as a substitute for Emanuel Rivas in the 69th minute of the match. His performance was well received by the press. Despite his early debut, he was not selected again by Ruggeri, and his replacement Osvaldo Sosa, for the remainder of 2002–03 season, having made only one appearance in the Torneo Clausura. Following the arrival of coach Jose Omar Pastoriza, seven months after Agüero's first match he returned to the club's first-team in 4–2 win against Peru's Cienciano during the group stages of the 2004 Copa Libertadores. This meant that he also became the youngest player to participate in the Copa Libertadores, a record that he held for three years. One month later, Agüero again featured in a Copa Libertadores fixture against Ecuador's El Nacional. On 19 June, for the first time, Agüero played a full 90-minute match for Independiente against Atlético de Rafaela during the Torneo Clausura. He scored his first goal for Independiente in a 2–2 draw against Estudiantes on 26 November, with a 22nd minute shot from outside the penalty area. Agüero became a regular in the club's first-team, being selected for the Argentina U-20 squad for the 2005 FIFA U-20 World Cup, which Argentina won.