As far as getting riding time and experience, ESPECIALLY at your age, it's best to go by word of mouth. I have met SO many horse people, and have been asked to ride more horses than I could ever ride- even if I didn't work full time!! Now, I don't ask for any money in riding, in fact relieved just not to be PAYING to ride!
As far as jobs in riding goes, yes, they DO exist, but you usually find out about them at the most unexpected times, and they can be demanding-can be a lot of pressure, and can demand to travel far from home for long chunks of time. Basically, a lot of them just would not be right for you yet. I have a friend who got hired to condition endurance horses, he was recruited at a competitive trail ride, by talking to the right people. I know people who have gone out west and worked at ranches, as well.
Anyway--the volunteer idea is a good one! You're still young, many possibilities ahead :)
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.