Complex sentence: He scored 13 goals in 2002--03 (before his 21st birthday) and soon became a star in Porto's frontline, alongside Derlei; he had also earned a permanent selection for the Portuguese under-21 team by this time.
Split into simpler sentences: He scored 13 goals in 2002--03 (before his 21st birthday) and soon became a star in Porto's frontline, alongside Derlei. Postiga had also earned a permanent selection for the Portuguese under-21 team by this time.