Aguiar was a member of the Portugal under-21 team that finished third at the 2004 UEFA European Championship and qualified for the Athens Olympics, along with several future full internationals such as Hugo Almeida, Bruno Alves, José Bosingwa, Danny, Raul Meireles and Hugo Viana.
Aguiar was part of the Portugal under-21 team that placed third in the 2004 UEFA European Championship and qualified for the Athens Olympics alongside several future national team players like Hugo Almeida, Bruno Alves, José Bosingwa, Danny, Raul Meireles, and Hugo Viana.