Hewitt and Roddick weren't exactly old when they started being passed by the current generation in the rankings.
Uh? Andy was passed by whom exactly?
Roddick was still making quarters and semis as the younger generation came into the scene, not to mention his Wimbledon final. Federer remained his primary obstacle.
He has winning records over most of the current crop, including Djokovic, Berdych, Tsonga, Almagro, Gasquet, Isner, and co. He has respectable records against both Nadal and Murray. And he even scored wins over rising players like Dimitrov, Raonic, Dolgolopov, Nishikori, and Tomic prior to his leave.
And if that weren't enough, he split major meetings with each of Djokovic, Murray, and Nadal (both their meetings outside of their respective primes).
The idea that he was passed up by this group is a myth. Federer was his only real career hindrance.
You could even argue that Hewitt wasn't necessarily left behind so much as he was brought down by unfortunate injuries and poor draws due to a helplessly declined ranking.