I have a confession to make.
I'm 34 years old, and I write fanfiction. And I like it.
Since I also write original fiction, generally when I tell people I write fanfiction, they look at me sort of sideways and say something to the effect of, "You know you won't get paid for that, right?" Well, that's not why I write fanfiction, and I don't think doing so demeans me as a writer.
You know you've read fanfiction that utterly rapes the Canon on which it is presumably based, twisting established characters in ways that would make their creators cry and using spelling and grammar that would give Zalgo a headache. Heck, I've read stories that made me honestly wonder if English was the author's primary language, or if they'd even read the books, seen the movies, or understood the Canon.
I've also read some amazingly good fanfiction that takes the story in a new direction without violating the internal laws of the original material. I've read fanfiction that explored the implications of concepts that the author felt were left inadequately explored in the original material, that deconstructed and reconstructed everything from a single character to the entire franchise, and it was awesome.
To be fair, I've read original fiction that made my eyes bleed and my brain throw up as well, alongside the perfectly enjoyable novels and short stories that populate my library--meaning that getting published is not necessarily the mark of a good writer.
A good writer--regardless of the medium--can make the reader care what happens to the characters. A good writer can make the reader laugh, or cry, or hide under the bed (for all the right reasons), and make you want to follow the story to the end to see what happens. A good writer can immerse a reader in the story's world, and make it feel like a real place. This is true whether their work can be found on the shelf of a library or in the pages of Fanfiction.net. Writing fanfiction does not automatically make you a bad writer--it makes you a fan of the source material.
And as with any sort of writing, fanfiction can allow a writer to practice their craft and improve themselves if they allow it. So write on, fans!
I have another confession to make.
Blue Sky made me cry.