I just had a 17-year-old Less Wronger e-mail me for advice regarding the Thiel Fellowship after reading my application essay from last year when I was 19. We had a long instant message conversation where I gave him a lot of advice which he seemed to find highly useful (my biggest piece of advice was to start teaching himself programming using Learn Python the Hard Way, shamelessly asking for help using a pseudonym on IRC channels, forums, and Stack Overflow if he got stuck).
It seems likely that there are other Less Wrong users who still live with their parents who could benefit from life and career advice. I'm especially interested in reaching those who see reducing existential risk as a major life goal.
A related idea is for people who have some goal they want to achieve, like having a romantic relationship with someone of their preferred gender or being admitted to a prestigious graduate school, to pair up with someone who has accomplished that goal.
So if you're a young person who would like advice, an older person who would like to give advice, a person who wants to accomplish a goal, or a person who has accomplished a goal and is willing to help others accomplish that goal, consider leaving a comment on this post so you can find your counterpart.
I realize this post is a bit open ended--consider it an experiment in tapping Less Wrong's social capital in a novel fashion.
Talk to your high school's guidance counselor. They should have a few aptitude tests that you might find useful (like this short one).
Think of a career as the way in which you provide value to others. The most important word in that sentence is 'you, then 'others'. Figuring out what your personality is and what your mental skills are is the best springboard. (And mental skills are way more than "g"- there's a lot of variation in mental architecture among people.)
I've taken tests like that in the past. They seem useful for narrowing things down, but what frustrates me is that they make recommendations based on things you shouldn't be using to make long-term recommendations. For instance, the test you linked to asks about level of interest in art, business, law, computers, etc.. I don't want to spend the next 50 years of my life in neuroscience because I happened to have a crush on biology in high-school.
I mentioned g because it's important for career choice and it seems unlikely to change much, unlike other things like social skills, programming skill, or self-control, which can be learned (and should be learned if some brief learning is needed for a long-term better career).