Gross Calls String Theory A Bizarre Science That Is A Dangerous Business
IMO, age has everything to do with it. You rarely see physicists accomplish much after their thirties (other than perhaps Richard Feynman, but even his accomplishments slowly declined in the end). I suspect it has more to do with psychology than pure reason. It is this very experience and rigidness, I believe, that makes the older physicist able to tell when a theory is correct.
It didn't exactly come out as fluent as I had hoped, but I hope you see what I am trying to say.