Not sure who the youngest scorer is, but my friend tells me that the youngest ever World Cup player was Norman Whiteside in 1982 in Spain, he played for Northern Ireland (yes, somehow they made it to WC), he was only 17 at the time.
Pele, 1958 World Cup, 17 years and 249 days. Scored his MOST FAMOUS goal against Sweeden. This is the precise answer. Pele had been however playing for the Brazilian National Team since 1957, when he was 16 but those games were not World Cup Game. To see the goal click here: http://www.youtube.com/watch?v=m15yryxTXCs or youtube.com words "Brazil" "Sweeden" "Pele".
former soccer player (in Brazil) and curious about soccer facts. City of Pittsburgh, USA.