The trio have won every Grand Slam since 2017, with 33-year-old Nadal claiming the latest title at Roland Garros on Sunday, dispatching 25-year-old Austrian Dominic Thiem in the final for a second straight year.
Becker, who won Wimbledon aged 17 and went on to win six Grand Slam tournaments, said it was the mental aspect of the game more than anything that set Nadal, Federer (37) and Djokovic (32) apart.
"I was just reading a stat that no active player outside the big three under 28, apart from Thiem, has been in a Grand Slam final," Becker said looking ahead to Wimbledon which begins on July 1.
"That's not good, that's not a compliment for anybody under 28. And don't give me that 'the others are too good'. We should question the quality and the attitude of everybody under 28, it just doesn't make sense."
Becker added: "As much as I respect Roger, Rafa and Novak, young players should show up. Eventually, they will be too old, but you want to see the passing of the torch while they are still in their prime.
"There's a certain mentality that they (younger players) don't have, that the three others do have. It's not the forehands, it's not the fitness. It's mindset (and) attitude that makes the difference between winning and losing."