[pacificsun]

If you are turning 18, why not try to get paid for your open-source work: https://developers.google.com/open-source/gsoc/

Most small companies have enough to worry about paying full-timers, let alone take on interns. If you are very ambitious, you can try becoming a contract developer (other people here are better-equipped to talk about that in detail).

[applecrazy]

How would an underage contract developer get contracts in the first place? Most SV companies are very exclusive about who they hire.