I'm 13 years old and I'm writing a story called 'Destiny'. This book is a fanfiction of the TV-Show "Merlin" (the TV series 2008) and I've added a character into it. There currently is no book of Merlin, so would it be okay to publish this book or would I have to get permission from the directors and creators of Merlin? It's not complete yet, but I really want to publish a book on it one day.
Fan fiction is a legal gray area, but in general these days, most shows will ignore it UNLESS you are trying to make money from it in some way --in which case they will definitely come after you (lawyers always follow the money!). That means publication as an actual novel is out. It's also 100% sure you would be denied permission if you did ask, not because you're a kid, but because when they get ready to publish a book, they will hire an established author.
On the other hand, however, most popular shows have active communities of fan-fiction writers and readers. Some shows actually even encourage those communities. If you can find a community like that, you'll have a place to post your work, get feedback, and maybe even build a fan base of your own. You might even have more people reading your work than if it was a book.
I don't personally enjoy fan fiction, but millions of people do, and some very big and famous writers got their start that way. So keep on writing, share your work with other Merlin fans, and you'll be well on your way to writing an original novel with your own all-new characters and setting one day.
Of course you would need the permission of the creators. A fanfiction is always based on something existing and you are not allowed to earn money with it. Only if the creators allow that.
And allow me to say that: With 13 most of my stories were underdeveloped and hadn't shown any sign of something, that could suck the readers in and hold them.
As hard it is to say (but you wouldn't ask if you wanted to be sugarcoated), but why should an million dollar producer, pay a 13 year old to write a story, if he could get Stephen King for example. As long as you didn't make a name as writer, have connections into the field of work or are related to some famous writer, the chances are very near to 0 to publish a fanfiction to an existing story.
There is a writer, who got famous with a fanfiction (E. L. James), but even she changed the characters, places and names as she published the story as book.
What do you mean by Merlin? The legendary character of Arthurian legend who is in the public domain and anyone can write and publish story about him:
Or specifically from a TV show or story featured on the list? That might be more complicated...you might have to read up on copyright law from your country.
Whether your fiction is good enough to be published is another matter.
Edit: If you do indeed mean the 2008 series, then what might help you determine if you need permission is actually reading up on the stories of Merlin that the show is based on(start with the Wikipedia article and go from there). That way you can determine what aspects of your story belong to the TV writers and which ones are fair game and are just part of the Merlin myth which is in the public domain.
The short answer is NO, you don't have to ask for permission. Merlin, and the characters associated with him, were created in around the 1680's according to this site (a very interesting read - with video included):
If you are taking from a series, for example using Thor and Loki directly from the Marvel Universe, then maybe need check. Characters can be trademarked, but the names themselves cannot be trademarked because they've appeared in much older works. Another example is Tolkien's characters names, taken directly from mythology (although the characters are different).