In Canada, you can actually write a letter to Santa and he will write back! This service is provided by Canada Post. I tested this service 7-8 years ago. I couldn't believe it when I received a reply.
Even though I am too old to believe Santa actually exists, receiving a letter from "Santa" brought joy to my heart.
Kids under the age of 10 believe in Santa Claus. It is hard to say when they come to realize that it is actually their parents putting all these presents under the Christmas tree. It seems all the adults are trying very hard to protect the innocence of children. If you google "Does Santa Claus exist?", you won't find a definite answer. That's because nobody wants to spoil the fun.
Some years ago, I was influenced by my Christian friend. She is very religious, and taught her daughter that Christmas is about Jesus, not Santa Claus. I made the dreadful mistake of telling my then 7 year old son that Santa Claus didn't exist. When I saw the horror and disappointment on his face, I tried to take back what I said, but it was too late. To this date, I still regret doing something so horrible. I am not a Christian and neither is my son, so why was I so stupid?
My 2 younger children are 8 and 9, and they still believe in Santa Claus. Maybe my 9 year old son is doubting Santa's existence, he keeps asking questions like how Santa enters our house, since we don't have a chimney and we turn the house alarm on, etc.
I tell them, "It is magic.. you just have to believe it."