Yeah, I don't know of any places who hire under 16 either, and they really prefer 18. I had a heck of a time getting a job at 16 because most places didn't like my age. I eventually found work with Cherrydale, and I collate (organize, staple, sort, and envelope fundraising material for public schools) for them. I don't get paid by the hour, I'm paid by the envelope. I get $0.10 per envelope, but I'm quite fast and can do about 6-7 envelopes per minute, so I'm paid enough to support my horses as long as I budget carefully. I also tutor, babysit special needs kids, and house/dog/horse sit for people to supplement my income. Maybe you can do one of those things?
My rule of thumb, is you need to be making enough money to pay for monthly board/farrier/feed costs, plus an extra $100 ideally, in case you need something. You should also put money away for emergencies. Again, ideally you'd have at least $300-400 for emergency. I can't say much though because at the moment I'm making just BARELY enough to board/feed/farrier my horses, and have little for emergencies in the bank. However, I'm also paying for my car insurance which is not cheap, and gas which is not cheap either. I wouldn't recommend it though...its stressful.