Jesus was slain from the foundation of the earth (Revelation 13:8).
If Jesus, while He was 2 years old, had been killed by Herod when he tried, His blood would still have been effective at paying for the sin of the world. Yahweh was merciful, and spared Him to a fullness of time – when He had taught people how to lead as servants; when He had prepared people to become aware that He had died to make them His family in the earth, empowered to rule and to serve and to bless as His Kingdom of priests and His holy nation.
What Jesus declared was finished on the cross was the payment for sin. What the wise men declared was begun was the increase of His Kingdom, which will never stop, as Isaiahprophesied in 9:6&7.