Fredo was only a few years younger than Tom. If Tom and Ola knew each other from "the old days," so would Fredo. And, given the close association between Vito and Roth, Fredo would have known Roth, too.
As far as the movies are concerned, is there any reason to think this, beyond the fact that Fredo attended the meeting with Sollozzo and so could be expected to attend other major meetings? (Fredo in the book, of course, was much more closely connected to the family business).
With that as background, I would guess that Fredo was included in the Sollozzo meeting because it was ostensibly a new matter that could bind the family and require a degree of formality. If Sollozzo's offer was accepted, Fredo may not have been included in any discussions involving operations. Similarly, Fredo may not have been privy to operations involving Roth, because those had been handled for years by Ola and Genco, and then passed on to Tom.
