Herod began his actual reign in BCE 37 upon the death of Antigonus II that ended the Hasmonean Dynasty.
Josephus makes it clear that Herod's temple had been started in Herod's 18th year, counting inclusively from the year beginning his reign.
BCE 37; Herod starts reign
BCE 20; Herod begins temple
27 CE; Jesus' first Passover during His ministry, being 30 1/2 years old
See Antiquities XV 11
From BCE 20 to 27 CE is 46 years, counting from year 20 to 19 as 1 and ignoring the 0 year that didn't exist.
John 2:20 Then said the Jews, Forty and six years was this temple in building, and wilt thou rear it up in three days?
So, Christ was 30 1/2 years old and Herod's temple was 46 years old at the time when this exchange took place in Jerusalem at the Passover.
Christ of course was referring to His Body at its death, burial, and resurrection as the temple that will be raised.