Much like manatees being mistaken for mermaids, this has some basis in fact. The way that it seems to work is that an incompetent person sees a teen perform some miracle of computing - changing system settings, troubleshooting a common network problem, etc. So, since this kid was able to outdo them, the kid must be A) Brilliant (so he could figure it out when they couldn't) B) Highly skilled (because admitting that anyone with a reasonable amount of knowledge could do it would be admitting to ignorance). So then this person tells a friend about this 'genius', which is combined in the friends mind with tales of 'hackers' who can break into computers. And there you have it - the teenage genius hacker.