What the research actually says, why the range is so wide, and how to tell where your child is.

Now, let me tell you something, dear. This is the question parents ask me most, and they usually ask it quietly, as though it is slightly embarrassing to want to know.
It is not embarrassing at all. You are trying to work out how much time you have left.
Most children stop believing between seven and nine. Studies over several decades put the average around eight.
But averages hide the interesting part. Plenty of children are certain at ten, and some quietly work it out at five. Neither is a problem.
Belief does not end because a child reaches a certain birthday. It ends when the evidence around them stops holding together, and that depends almost entirely on circumstances.
It rarely arrives as a single announcement. What you usually see first is testing.
A child in this phase is not asking you to end it. They are asking you to defend it, which is a different request entirely.
There is a long middle period, often a year or more, where a child half-knows and chooses not to look directly at it.
This is completely normal and it is where a great many eight-year-olds live. They enjoy the tradition, they suspect the truth, and they have decided the tradition is better. Let them have it. There is no obligation to force clarity.
The rule I would give any parent is this: answer the question they actually asked.
A child asking is Santa real at five usually wants reassurance. The same words at nine, asked seriously, in private, usually mean they already know and want to be told the truth by someone who respects them.
The difference is in the tone, not the words. You will know.
Not as much as parents fear. Children who work it out early are usually rather pleased with themselves, and they very often become enthusiastic co-conspirators for younger siblings.
What upsets children is not the discovery. It is being laughed at for having believed, or feeling they were deliberately made a fool of. Handle those two things well and the discovery itself lands softly.
If your child is seven or eight, you may be in the last season where the whole thing is uncomplicated. That is worth knowing, because it changes what is worth the effort.
The moments children remember are the specific ones, where Santa knew something he could not possibly have known. A short call where he uses their name and mentions the thing they have been working on is the kind of memory that survives long after belief does.
Book a live call with Santa while it still means everything to them.
Seven to nine, on average, with an enormous amount of variation and a long fuzzy middle where they half know.
Do not rush it, do not force it, and do not be too sad about it. The children who are handled kindly through this become the adults who do it beautifully for their own.