The question about when children should stop believing in Santa can be tricky. Not only is every child and family different, but, as parents, we want to make sure that we're walking the line between encouraging their imaginations and helping them grow up. While it's understandable for 8- and even 9-year-olds to still believe in Santa Claus, still doing so at 10 is just downright silly.
At 10, children become tweens. Ten-year-olds should be getting ready for things like junior high, school dances, having braces, and listening to embarrassingly bad music at an unhealthy volume. They shouldn't believe in a large man and his magical elves that bring gifts from a place called the North Pole once a year.
Children, for the most part, believe what they're told. At 10, they should be developing keen inquiry skills. Being able to question the world around them is necessary in school and in life. It's what creates divergent thinkers and problem solvers and helps them develop a keen sense of self. Believing in Santa at an older age demonstrates less about retaining childhood purity and more that they're refusing to think about the world in an inquisitive and thoughtful way.
As a parent, I get the desire to slow down time. Not believing in Santa is partly an acceptance that your child is growing up, and obviously that's hard. Yet continuing the Santa ruse at a certain point becomes less about the child and more about the parent's needs. There are other ways to retain their sense of childhood wonder without resorting to folklore best reserved for preschool-aged children. Try collecting toys for other children or taking them to see the lights in the city. Things like that will still encourage childlike awe.
And if your kid is 10 or older and still believes in Santa, you can always break the news to them in a gentle way. Try encouraging their reasoning skills by asking them questions about the logic of Santa. Hopefully, once they realise all the inconsistencies, they'll recognise that the magic of the holidays doesn't actually come from the North Pole, but instead from themselves and their families.