The Lord's prophecy has dual fulfillment and probably applies to Isaiah's son. The destruction of Syria and Ephraim are to come before this child reaches the age of accountability.
"There are plural or parallel elements to this prophecy, as with so much of Isaiah's writing. The most immediate meaning was probably focused on Isaiah's wife, a pure and good woman who brought forth a son about this time, the child becoming a type and shadow of the greater, later fulfillment of the prophecy that would be realized in the birth of Jesus Christ. The symbolism in the dual prophecy acquires additional importance when we realize that Isaiah's wife may have been of royal blood, and therefore her son would have been royalty of the line of David. Here again is a type, a prefiguration of the greater Immanuel, Jesus Christ, the ultimate son of David, the royal King who would be born of a literal virgin. Indeed, his title Immanuel would be carried forward to the latter days, being applied to the Savior in section 128 verse 22 of the Doctrine and Covenants." (Christ and the New Covenant, 79 as taken from Commentaries on Isaiah in the Book of Mormon, ed. by K. Douglas Bassett, [American Fork, UT: Covenant Publishing Co., 2003], 130-131)