I published my first book at seventeen. At my very first booksigning, a gentleman looked at my book and its title and asked me "what do you know about revenge?". It was an innocent question, but being young, inexperienced, and frankly naieve, I did not have a very good answer for him.
People still ask me questions like that, what do I know about the topics I'm writing? After all, I'm young, new to the industry, to writing itself. My books deal with fairly heavy human issues such as war, genocide, death, loss, revenge/how to cope with anger, love, fate, even our constant search for something greater than ourselves.
I think the real question is, where would we be if we wrote only what we knew? If Tolkien or Lewis or Asimov only wrote the things they knew, then this world would be dare I say boring. There would be no Orcs, no Trolls, or Elves, no Walkers, no Dragons, no Grumkins.
So don't be afraid to go crazy. Write whatever you want. Write about a world where dragons ride giant spiders and fight over a throne made of pancakes. Write about a hero who has to defeat the invading spaghetti monsters with the Colander of Justice. Don't let anyone or anything tell you that you cannot do something. Leave the boring for the real world, write something that you want to read. Write what you don't know.