Scrum-half Youngs and full-back Tonks are in the 26-man group for the tournament, which takes place in Japan in June.
Youngs, who has played 16 times and scored three tries for Tigers so far this season, is one of 16 players with Guinness Premiership experience this season. He is also one of eight survivors from the 2008 campaign when England were beaten in the final by New Zealand.
England Under-20 head coach Mark Mapletoft said: “"This is an exciting squad to be involved with and we have the talent, experience and ambition in the group to mount a really strong challenge for the world title in Japan this summer.
"Everyone in the squad for this tournament has made an impact in the Premiership this season or on dual-registration with National League clubs and that made selecting a squad of 26 a very tough task. There were close calls and there will be some very disappointed players out there.
"That said, it's a pleasure to welcome back the likes of Courtney Lawes, Graham Kitchener, Ben Youngs and Charlie Sharples, who all only appeared once or twice for us in the Six Nations before getting a lot of exposure for their clubs, and to have Dave Lewis returning after injury."
England start the tournament with Pool B games against hosts Japan on June 5 (7pm), Scotland on June 9 (5pm) and Samoa on June 13 (1pm), all at the Prince Chichibu Stadium in Tokyo. The semi-finals and finals are scheduled for June 17 and June 21.
England Under 20 Junior World Championship Squad:
Forwards
Bob Baker (London Wasps)
Calum Clark (Leeds Carnegie)
James Clark (London Irish)
Carl Fearns (Sale Sharks)
James Gaskell (Sale Sharks)
Jamie George (Saracens)
Graham Kitchener (Worcester Warriors)
Shaun Knight (Gloucester Rugby)
Courtney Lawes (Northampton Saints)
Joe Marler (Harlequins)
Ben Moon (Exeter Chiefs)
Josh Ovens (Bath Rugby)
Dan Williams (Gloucester Rugby)
Chris York (Harlequins)
Backs
Tom Casson (Saracens)
Rory Clegg (Newcastle Falcons)
Luke Eves (Bristol Rugby)
Tom Homer London Irish)
Dave Lewis (Gloucester Rugby)
George Lowe (Harlequins)
Rob Miller (Newcastle Falcons)
Charlie Sharples (Gloucester Rugby)
Seb Stegmann (Harlequins)
Henry Trinder (Gloucester Rugby)
Greig Tonks (Leicester Tigers)
Ben Youngs (Leicester Tigers)
