I don't remember when I stopped believing in Father Christmas, but there was a time when I believed very strongly.
According to the story, and I was far too young to remember it myself, I was about 2 yrs old and my sister would have been about 4. We went to a Christmas Party on camp and Father Christmas himself was there! When he arrived, I assume we children were not expecting him, our eyes were wide with excitement and awe. He gave us presents and I received a white poodle pajama case that I had for years. It seems it was my favourite thing for a very long time, but at about 8 or 9 I decided I did not like it very much. A poodle seemed a bit childish and it was so much easier to just put my PJs under my pillow.
A couple of years ago, the poodle PJ case come up in conversation and I mentioned that I could not imagine who would have given me such a thing and that surely I never actually liked it. That was when my Mum told me the story of the Christmas party, the poodle and Father Christmas. The really surprising part? My Dad played Santa that year! It seems unbelievable to me now, but neither my sister nor I recognised him as anything other than Father Christmas.