When and how did the Triune God determine the plan of the ages?
The plan of the ages was determined by the Triune God in eternity past, before time began. In the Godhead's counsel, the Trinity settled it in a split second, not by lengthy debate, and in foreknowledge saw every detail of history unfold.
Les teaches that God has had a plan for the ages from eternity past. Using Acts 2:23, where Peter preaches on the Day of Pentecost and speaks of Jesus as delivered by the determinate counsel and foreknowledge of God, Les explains that somewhere way back in eternity past, the Triune God came into counsel. He notes they did not have to sit for an afternoon batting the matter back and forth; rather, in a split second, they determined the plan for the ages. In His foreknowledge, God could see every detail of it coming down through history, and everything falls in place exactly as He predetermined, even while men and nations act under free will.
Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain: