The timeline was somewhat confusing around this time period. Voldemort had already created the Locket Horcrux, quite a few years before. However, he wouldn't put it in that cave until Regulus Black became a Death Eater. That would be... this year. Sirius had been disowned the summer before his sixth year, and Regulus was forced to become a Death Eater in his place. Regulus was in his fifth year. If he was fifteen yet, it could only be recently...