So.... Mister Crow...
"We all say that a true goth knows the history or knows what the subculture is about. Yet in everyday life people get judged by their appearance, and no one wants to find out what they actually know about the subculture. So anyone can just put of the attire and fool half the people easily."
To be fair, if they can be fooled then their opinion on goth doesn't matter. People who judge on appearance are the people we do our best to ignore, since they haven't a clue what they're on about. A real goth is always different from a babybat. To a real goth, the music, the history, the clothing are all second nature. Babybats generally look like they fell off the rack at Hot Topic or (insert generic alternative clothing store here), and won't know most, or even any, of what a full-fledged goth's talking about. Your age won't matter if you're truly into goth. It's whether you've got a basic grasp of the subculture and you look comfortable in your own skin that matter.
And if your mother has issue with you becoming a goth, direct her to this video from the BBC's Culture Show, which paints us in a positive and accurate light, and do remind her that a very high percentage of goths go on to higher education and very good jobs.
http://www.youtube.com/watch?v=QZXSMLoG0fw
(just as a by-the-by... I know the girl with the red dreds about a minute into the video. She's a lovely woman from around Leeds. These are all real people in the video, all real representations of our subculture.)