Which players won both FIFA U-20 World Cup (until 2005 known as the FIFA World Youth Championship) and FIFA World Cup?
This would show players who excelled both in the young age and in their later career.
I guess that there will not be too many such players but, of course, I might be wrong.
I did not find any source where I could systematically search for statistics like this. By some more-or-less random searching I was able to find Maradona, Dunga and Xavi.