I have to call bullshit on that whole idea. That's a fundamentally age-ist viewpoint, and I would actually pretty strongly disagree about Scott's more recent filmography. I think Kingdom of Heaven DC is outstanding, I think Black Hawk Down is a fucking impressive film-making accomplishment, and American Gangster is way better than people give it credit for.
Clint Eastwood has had many misses as a director, but he's also made several excellent films in the last 20 years.
Woody Allen has had plenty of good late career films.
I agree that there's certainly an aspect of a "hungry" young film-maker trying to prove themselves that results in a higher success rate, but it's not a firm rule by any means.
Reading the interviews with Ridley/Lindelof, it's clear to me that Lindelof/Spaights were the weak link on Prometheus, not Ridley. Ridley basically threw out a bunch of ideas he thought would be interesting (and I think they are, personally I think the whole Space Jesus thing is a really ballsy way to go and a neat sci-fi concept), and had Lindelof write a script around them. The flaws here are "connective tissue/dialogue" more than the concepts themselves, which is the fault of the screen-writer. Furthermore, the design and technical aspects of the film are absolutely outstanding, so you can't tell me Ridley's lost it or wasn't giving it his all on the project. If he's to blame, it's for approving the script, and not hiring some-one else to take a pass on it and refine the dialogue.
If Blade Runner 2 is being written by the original writers of Blade Runner 1 (which I believe is the case), then I think that movie could still turn out absolutely great.