In 2016, Curbelo won a gold medal with Puerto Rico at the Centrobasket Under-15 Championship in Patillas, Puerto Rico after averaging 16.5 points, seven rebounds and 6.5 assists per game. At the 2017 FIBA Under-16 Americas Championship in Formosa, Argentina, he averaged a tournament-leading 21.8 points, 7 rebounds, 3.2 assists and 3.5 steals per game and won a bronze medal. Curbelo competed at the 2018 FIBA Under-17 World Cup in Argentina, where he averaged 13.6 points, 5.6 rebounds and a tournament-high 5.9 assists per game and led his team to another bronze medal. He averaged 11.1 points and 5.6 assists per game at the 2019 FIBA Under-19 World Cup in Heraklion, Greece as Puerto Rico finished in sixth place.