A child who came of age after Pesach Rishon does not bring a Pesach on Pesach Sheini, according to Rabbi Nossan. Pesach Sheini, is a redressal for the Pesach Rishon. Since the minor was not obligated in the Rishon, there is no reason for him to bring the Sheini.
When Tisha B`Av falls out on Shabbos and is pushed off to Sunday, is a minor who came of age on Sunday obligated to fast?
The Divrei Malkiel talks about this. He first says that similar to our Gemora, since Tisha B`Av was supposed to be on Shabbos and on Shabbos he was a minor he should not be obligated. Sunday is merely the redressal of Tisha B`Av that falls out on Shabbos, like Pesach Sheini is merely the redressal of Pesach Rishon.
However, he then argues that since Chazal knew that Tisha B`Av would come out on Shabbos, they must have made Tisha B`Av on Sunday in years like that.