Germany won the U-21 Euros in 2009 with a starting eleven that featured six players who lifted the WC in 2014 (and interestingly enough, Fabian Johnson who Klinsmann got to switch to the USMNT at the senior international level). That's a damn good translation of youth team success to senior national team success. Most often, though, the translation isn't that pronounced.
Past winners and semifinalists of recent U20 WC, for what it's worth....
2017- England d Venezuela. Italy d Uruguay for 3rd.
2015- Serbia d Brazil. Mali d Senegal for 3rd.
2013 - France d Uruguay (Pogba was the French captain). Ghana d Iraq for 3rd.
2011 - Brazil d Portugal. Mexico d France for 3rd.
2009 - Ghana d Brazil. Hungary d Costa Rica for 3rd.
2007 - Argentina d Czech Republic. Chile d Austria for 3rd.
2005 - Argentina d Nigeria (some kid named Messi was in the Argentina side). Brazil d Morocco for 3rd.
Just because I was curious, I looked to see how Spain U20s fared in the late 90s and early 00s. They won in 1999 with Xavi and came second to Brazil in 2003 with Iniesta.
So what does that all mean? Brazil a lot of talented young players. Beyond that, U-20 translates to senior level success sometimes (France, Spain) but more often does not.