Ask HN: How much to charge?

I learnt programming, web development and all IT things myself. Lately, I have started to work as a freelance. I'm 19 years old, I started programming since 12, I was and I still an amateur developer.

So my question is "How much to charge?" (the hour), right now I did few jobs (only 2) ($30/hour for a small job at Odesk and $120 for a 2 hours task). Last week, I got a request from a company to review and rewrite (some technical papers) for them. I told them my rate is $25 and they were happy with it!

I have stumbled upon few websites discussing it. M.I.T students are getting an average $20/hour for programming (http://web.mit.edu/sfs/jobs/wage_guide.html); so I thought $25 will be good considering my age and qualifications (no qualification rather than a small portfolio and apps I made). Also average freelance workers (freelance sites like Odesk) is $10.

So am I underestimating my capabilities? How can I measure my rate? If age doesn't matter, how can I evaluate my experience and charge accordingly to the market rates.

I fixed my rate for that job; but it seems that another startup may be interested on some JavaScript debugging (as I have similar experience to their issue, some event bubbling). This is why I posted this question. It seems that some freelancers are charing $100 the hour and even more. Why? Is it portfolio related? How do you set your rates?

Thanks for the advice.