I learned the deception I think when I was 7. Santa in those days would come to our house on Christmas Eve and deliver our presents in person. Usually played by an older cousin or relative. I was never phased by the fact he drove up in a car or had one of my cousins as a helper.
But one year I had a view through an angle where I saw my dad hand off the Hefty Bag of presents to “Santa.” Illusion shattered. But hell, I was still getting presents, so it didn’t bother me much.