I had our Granddaughter believing at 10 years old in Santa!!
My son was perhaps 6 - he got up one morning and my wife remembered she hadn’t left money for his tooth. So she rushed into his room then hopped back into bed. A little bit later she hears “Hey - I checked when I got up and my tooth was there. And NOW there’s money!!” My wife told him she was the tooth fairy.
Several weeks later he is trying to fall asleep. Then she hears giggling coming from his room.
“What are you giggling about?”
“You know how you are the Tooth Fairy?”
“Yesssss?”
“I bet you’re really Santa Claus too!”
He was pretty proud of himself figuring that out. And then being in on the secret with his younger sisters.
In fifth grade they (twins) questioned Santa based on conversations with their friends and asked us about it.
“Do you think Mom and I would buy you all of that stuff!!??”
“They both looked at each other and big smiles came over their faces - “No WAY!”
The following year though the one daughter figured it out, and my wife was confronted by the other one just before Christmas. When my wife told her she was pissed. At the age of 17 she still remembers it. Pretty sure she has gotten over the ordeal thought.