Mosiah 6:4

And Mosiah began to reign in his father’s stead. And he began to reign in the thirtieth year of his age, making in the whole, about four hundred and seventy-six years from the time that Lehi left Jerusalem.

Mosiah became king when his father stopped being king. He was thirty years old when he became king. This was about 476 years after Lehi left Jerusalem.

Mosiah took his father’s place as king. He was thirty years old when he began to rule. This was about 476 years after Lehi left Jerusalem.