As fr self-doubt, I can't imagine any creative who does deal with that. But having self-doubt and being governed by it are vastly different things. The key is to use that self-doubt as fuel for doing whatever it takes to improve your understanding of the writing craft, understanding how the publishing industry works, and to have fun with the other fans of pop culture.
As to how old I was when I first published, that's actually complicated. I was 20 when I published my first magazine article; 33 when my first nonfiction book was published (a textbook on JUDO for Temple U.); 48 when my first novel was published (GHOST ROAD BLUES); 49 when my first short story was published; 51 when my first comic book was published ("GHOSTS", a Wolverine piece in the annual issue); 57 when my first short story collection was published (JOE LEDGER: THE MISSING FILES); and 58 when my first anthology as editor was published (V-WARS).
As to how old I was when I first published, that's actually complicated. I was 20 when I published my first magazine article; 33 when my first nonfiction book was published (a textbook on JUDO for Temple U.); 48 when my first novel was published (GHOST ROAD BLUES); 49 when my first short story was published; 51 when my first comic book was published ("GHOSTS", a Wolverine piece in the annual issue); 57 when my first short story collection was published (JOE LEDGER: THE MISSING FILES); and 58 when my first anthology as editor was published (V-WARS).