It’s Christmas Eve, and the scene is set. The tree is glistening, the stockings are hanging, the cookies and milk are waiting. The kids are sleeping soundly in their beds, while visions of sugar plums (and iPads and the new gaming system they’ve been asking for all year) dance in their heads. All that’s left is a visit from the big guy in the red suit.

For many children, Christmas is all about Santa Claus and his infamous trip down the chimney with his bag of goodies. Parents across the world scramble to fulfill wish lists to ensure the Christmas magic lasts as long as possible. But eventually, as it usually does, all good things must come to an end.

In a series of studies known as “The Santa Project” published by Dr. Candice Mills, a psychologist at the University of Texas in Dallas, research showed that disbelief in Santa begins creeping in for most kids around the age of 8. Her research also showed that some kids start questioning his existence at 3 or 4, while others hold on to their belief until 15 or 16.

The truth of the matter? The older children get, the more observant they become.

In 1936, child psychologist Jean Piaget proposed a “Theory of Cognitive Development” in children that consisted of four stages. Piaget stated that around age 8 (which is indeed the common age of disbelief agreed upon by psychologists), children enter the third, or concrete operational stage, of thinking. This is a stage where they begin to question impossible things. And we adults know without a doubt that while the idea of Santa is indeed magical, it is most definitely impossible.

I mean, really — we expect our kids to believe that one man travels the entire world in one night, leaving a gift for every single child. In a flying sleigh, no less. Led by a magical reindeer with a shiny red nose. Younger children can use their imagination to fuel this creative fantasy. But as they age, kids begin to accept less of the fantasy and start asking more questions that tiptoe around the truth.

So, how long did your child believe in Santa?

Personally, my son is 11 and is still a true believer. We don’t go as far as planting fake footprints around the Christmas tree, but we do wait to put out most of the presents until he falls asleep on Christmas Eve. In our house, he’s allowed to ask Santa for one present only, and we do our best to make sure he gets it. It’s wrapped or set up in an extravagant manner, so it’s the first thing he sees when he wakes up on Christmas morning.

Other moms I spoke to for this article revealed their kids hit that disbelief milestone much younger. Last year, at the tender age of 5, Hannah G.’s son got into a habit of asking questions about everything. When Christmas rolled around and he began seeing different versions of Santa in movies and on television, he asked her if Santa was real.

Hannah says she had already decided when he asked, no matter how old he was, she wouldn’t lie to him. So, she told him the truth. Her son accepted it at the time… but then chose to believe again this year. If he asks her again, though, she’ll tell him the truth. Again. (But for now, she will let him live in blissful 6-year-old denial.)

As pediatric psychologist Eileen Sosa, Ph.D., told Children’s Health, “It’s not an overnight shift in thinking, and there’s no set age where children should know the truth about Santa Claus.”

Sosa believes children begin to see the truth gradually, so that by the time you tell them your side of the story, they’re ready to listen. She also says several factors contribute to their ability to believe, including their family’s traditions, their cognitive development, meeting a realistic version of Santa Claus, and, of course, what other children are saying at school.

Cassie M. tells me her oldest son was 10 when he found out, but he was never given the chance to ask her if Santa was real for that very reason. He came home from middle school one day and said he knew Santa didn’t exist. Some of his friends had told him Santa wasn’t real, and he believed them. She admitted they were right and told him the whole story of jolly old St. Nick. To this day, he helps her keep the spirit alive for his younger siblings.

Some children don’t (and have never) believed at all, no matter what their ages may be.

And some parents simply aren’t comfortable with the one thing necessary to preserve the spirit of dear old Santa: lying to their kids. These parents opt to tell their kids the truth from the get-go rather than spend years trying to convince them that a jolly man in a red suit squeezes down the chimney to bring them presents every year.

Jennifer S. says, “Unpopular opinion, but my kids know Santa isn’t really real. They believe in him as the character of Christmas, but they know that’s as far as it goes.” She goes on to say that she did teach them not to ruin it for others around them since every family teaches their kids different things.

One upside of being honest with your kids in the beginning? If your child never believes in the first place, you won’t have to spend every Christmas wondering if it will be the last year you get to play Santa. Still, for those parents who look forward to creating Christmas magic, every year that passes brings you closer to the end of an era.

Whether your child is 4 or 14, there is no magical number that dictates whether or not they should still believe. There is one certainty, however — eventually, they will find out. And Christmas will be changed forever.