Drogon did disappear for a while at the end of season 5, so it's possible that while he was gone, he laid eggs. But Drogon is male, so that couldn't possibly work, right? Well, according to the Game of Thrones wikia, in A Feast For Crows, Chapter 35 , it is explained that Westeros historian Septon Barth believed that dragons were able to change gender and that they were "as changeable as flame" according to what a given situation called for. If Drogon laid dragon eggs back in season 5, those dragons would have had time to grow up a little, into the teen-like dragons we saw during Game of Thrones' middle seasons. Also... sort of like the image in the Thrones opening credits, of one large dragon and a handful of smaller ones. No?