According to the Merriam-Webster dictionary, philanthropy is an act or gift done or made for humanitarian purposes. Just because you are a teenager who does not have thousands of dollars to give away does not mean you can't be a philanthropist. West Australian based organisation Impact100 gives people the opportunity to put their money together with other people and choose which charities to give it to. They have a YoungImpact100 program for 12-18 year olds where the teens collect or save $100 to donate then attend a fun grant event where they pool their money with other teens and together choose a charity to donate the money to.
Generosity magazine has an article on other organisations in Australia with a similar purpose. You can find it here.