The first book I ever read with a gay character was when I was 13, the political novel Advice and Consent, written in 1959 in which a closeted gay Senator who had a "moment" in the Army during the War is blackmailed and commits suicide, not the most encouraging thing for a gay teenager to read right off the bat!
Next, when I was fourteen, I read Fire From Heaven, by Mary Renault about the young Alexander the Great and his love for Hephaistion. This was followed two years later by the sequel, The Persian Boy.