The average age at which [Christian] children stop believing in Santa Claus is 8, according to an informal poll of children and parents by an English academic.
As the Times of India reports, psychology professor Chris Boyle, from the University of Exeter, surveyed about 1,200 adults around the world to gauge their feelings about Santa Claus (or Father Christmas, or whatever he is called according to each individual's culture), when they stopped believing in him, how that belief affected their childhood, how it affected their own parenting, and so on.
The main takeaway from Boyle's research is this: As a parent, you can expect your child to believe in Santa Claus until about age 8. They may yet "pretend" to believe in the jolly gift-giver, either for their own sake (why risk giving up those glorious presents by not believing?) or for the sake of their parents, who are desperate to see their little ones cling to the magical belief. But generally, by third or fourth grade, Santa is relegated to the same position as the Tooth Fairy.
Still, about 65 percent of adult respondents said that, as children, they played along with the Santa thing for the sake of their parents.
Not for nothing, Boyle's research largely mirrors what's been known in the parenting, pediatric, and child psychology communities for decades: Kids stop believing in Santa by age 8. As Today's Parent reported in 2017, child psychologists in the '70s had figured out that, by age 8, most kids have worked out the math and figured that a single man can't deliver presents to all of the Christian kids in the world in a single night - at least, not without the help of their parents.