How does anyone know if your trying to be one or not?
You have a couple options:
1. Write a "debut" episode (about 30 pages of a manga story with a beginning, middle, and end. No "to be continued"s…) and send copies of it out to various manga-magazines (like Ribbon, Shocomi, Nakayoshi, Hana to Yume, etc…) to the "Henshuu-bu" (or "Editing Department"). If they accept your story and decide to publish it in their magazine (most people don’t get accepted the first time. I think it took Watase Yuu about three times…), then you will be working for that magazine and they will assign you to write more manga (usually short stories) as long as you are still popular among their readers.
2. Apply to be an assistant to an existing Mangaka. Some people who want to be Mangaka decide to be an assistant first so they can learn what it takes and learn how to draw better. An assistant of a Mangaka usually does the shading and backgrounds, erases pencil marks from the rough drafts, gets food for everyone (^_-), etc. you can apply to be an assistant by sending a letter to a Mangaka you want to assist, along with samples of your work.
Do you have to live in Japan?
You will most likely have to move to Japan (I’ve never heard of a Japanese Mangaka who doesn’t live in Japan…). I know they let Mangaka live in different cities in Japan (the magazines publish in Tokyo, but I know some Mangaka who live in Hiroshima, Osaka, etc.), but I don’t think the magazine you write for would let you live in another country.
Don't you have to write for a company?
It’s the easiest way to get your stuff published. If you write for a company, there are several restrictions on what you do or don’t write, how long/short your stories are, etc., but you will gain more popularity and more readers this way. Some Mangaka will go a little free-lance after they’re had lots of success and experience (like Watase Yuu with Appare Jipangu!), but if you’re just starting out, I’d say writing for a company like Ribbon or Nakayoshi is your best bet.
Does it matter what age you are?
I don’t think there is a limit, but I’m guessing you should be at least 18. Some Mangaka (like Sakura Momoko) made their debuts while they are still in High School (ages 16-18), but most wait until they are out of college.
How about if you don't speak the language or write in it?
That’s obviously a problem ^_^;; If you want to be someone’s assistant, it doesn’t really matter if you speak the language *well* or not (as long as you can understand "shade this in for me" and "could you get me a cup of coffee?" ^_-) if you want to actually write manga, though, you will need to learn the language. (I suggest taking a couple classes and living in Japan for a couple years…as a bare minimum. The grim truth is that no matter how much you study Japanese and no matter how long you live in Japan, unless you are extremely talented in linguistics, you will never learn Japanese well enough to actually write a literary work on your own. You will need to find a native Japanese person with superior language skills who can edit your dialogue and make it sound more "Japanese". There are several subtleties (speech patterns, word choice, proper sound effects) which your writing might lack if you don’t have a native Japanese person proofread your work.