The prophet pronounces that Samaria will be plundered before his son knows how to cry "my father" or "my mother"
Isaiah 8:4 ESV
4 for before the boy knows how to cry ‘My father’ or ‘My mother,’ the wealth of Damascus and the spoil of Samaria will be carried away before the king of Assyria.”
Samaria is plundered by the king of Assyria in the ninth year of Hoshea king of Israel which makes the son of the prophet between nine and twelve.
2 kings 17 ESV
17 In the twelfth year of Ahaz king of Judah, Hoshea the son of Elah began to reign in Samaria over Israel, and he reigned nine years. 2 And he did what was evil in the sight of the Lord, yet not as the kings of Israel who were before him. 3 Against him came up Shalmaneser king of Assyria. And Hoshea became his vassal and paid him tribute
By the time Samaria is plundered as a fulfilment of the prophet's pronouncement the boy is old enough (between nine and twelve) to cry "my father" or "my mother"
What did prophet mean that the boy would be unable to cry "my father or "my mother?".