Both of mine were almost ten when they stopped believing. In both cases it was caused by moving schools
DD, aged almost ten was in the top year at her primary school and with the rest of her class was given the task of writing to the younger children, pretending to be Father Christmas. She came home in floods of tears and was still grizzling on xmas day.
DS as a chorister had to board, so of course there were no surprise stockings. Instead he was told that we had to buy 20 small toys, one for each chorister, and they would be dished out between their stockings. He was far more philosophical about it, having correctly surmised that most of the other parents had more money than us so maybe he would do quite well out of it!
However, both of them say that they too would carry on the tradition if possible, because they loved having that belief in magic. The DGCs don't have a visit from Santa until they are at our house, because we have a chimney and my daughter feels that she wants them to have the same experience of sending their letters up the chimney that she had. It does rather sound from your experiences on here as if DGS's days as a believer are numbered, as he is six. He has quite a few older boy cousins, which won't help. I remember having to bribe one of my nephews to keep quiet.