Catherine: "In other words, it takes two decades for writers to be able to edit their own prose well." (see: Kellogg, R.T. (2008). Training writing skills: A cognitive developmental perspective [pdf file] Journal of writing research, 1(1), 1-26)
Had you said something like, "edit their own prose and get anything useful out of the process", I might agree. I don't think that most writers ever get to the point where they can "edit their own prose well." The best that you can expect is that if you give your work a week or two before you look at it with an eye to editing you can sometimes see poor writing of various sorts.
Oh, you might be able to do a good job of basic copyediting. But I think that significant writing is too closely tied to each writer's idiosyncratic thinking process for the writer to notice flaws of logic, organization, and flow as often as you really need to. And absent those things, I don't think you can say that the writer can edit his own work "well."
Frankly, finding even a professional editor that can do a good job of structural editing of the work of others is a non-trivial task. Even good editors are often lured down the alleyways of comma placement, spelling, and a random selection editorial peeves of dubious validity.
IME, a good reader who is willing to say, "I don't follow this part", is as useful as (sometimes more useful than) most editors. If you can tell me where you're having a problem, I can often see all or part of the problem and rewrite to correct it. If you can articulate why you don't understand the part, you're invaluable.
But good readers are hard to find, too.
On outlining: In many ways, the formal outlining taught in the 70s was worse than useless. Until I managed to get past the cumbersome process involved in the sort I was taught, I actively avoided outlining at all, to the great detriment to my writing.
Now, I always outline any writing longer than a page or two, but I haven't used a formal outline since the last time it was required in high school. I make notes, draw lines, and rewrite the parts I can no longer read for all the scribbling. At the end, I have a plan that is functionally an outline, but it bears little resemblance to the roman numeral, capital letter, lower-case roman numeral (etc.) outline taught in some writing classes.
This is why Writers Workshop makes no sense. Editing is a separate process from writing; in the real world (the 21st century world!) the professional writer has a professional editor.
I'm now convinced that the goal of writing instruction should be to teach students to write the best first draft they possibly can -- because the first draft is pretty much going to be their last draft even if you force them to "revise."
The fact is: they're not going to revise.
They're not going to revise because they're nowhere near being able to read their own words the way a stranger reads them.
The Psychology of Writing