Off the top of my head, Ronaldinho moved to Barca from PSG when he was 24 and went from 'extremely talented' to the world's best player. Rivaldo moved at 24 from Palmeiras to Deportivo, and then at 25 to Barca. Also went from talented youngster to one of the best players in the world.
I'm not sure what trend you're spotting? Am not exactly advocating signing Willian, but everyone doesn't need to go the Diego or Robinho way.
No but he seems to have more in common with the latter bunch, than Ronaldinho and Rivaldo.
Ronaldinho was 23 when he moved btw