How old do you have to be to purchase a lighter?

I live in Philly and i went into to 711 and there was a kid probally about 15 or 16 and i look prettty young but I'm 15 and i asked for a lighter he said i couldn't buy one. The person whom I believe was the manager was behind maybe he was just another worker but the kid straight up told me I can't. But from a gas station down the road they sold me and m friends a lighter. Is it legal and if so why didn't 711 selll me one?