I had a friend who used to deliver newspapers on his horse. It served multiple purposes - they were training for endurance riding, so the 10 mile route they covered was good training, and he made enough money to help keep his horse fed.
That being said - my friend was 16 at the time. A newspaper route like that is not something I would recommend for a 13 year old. He also worked at various small farms - he had a regular little route he covered to clean out barns and pens. It seemed like he was always working...or training...or competing.
Having horses is both hard work and expensive. Unless there is something you really need for your horse and your parents can take care of the basic necessities, why not just relax and enjoy your horse until your old enough to get a job?
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!