Prophets Musa and Harun were brothers.
My question is which one of them was the younger one and which one the older one?
Haroon (AS) was most likely older. In the Bible it says he was three years older and in Islamic history books it says one or three years older as Firaun killed the children of Bane Israel on alternate years, and Haroon (AS) was born in one of the years when he did not kill the children. In one place it is said that when the mother of Musa (AS) was presented for feeding him, the excuse made to Firaun for her having milk was that it was for his older brother.