I knew at a very young age, around 3, that there was no physical person who came into your house to leave presents. I told my mom this and she explained to me about the spirit of Santa and how he inspires others to give. We are not religious and so Christmas in not about Jesus to us, it is about Santa and giving people thoughtful gifts and being appreciative of what you got/have and what others do for you, it is about love and being with family and magic and miracles.
I always felt a little sad growing up that I was in on the whole Santa thing. My sister, who is 2 years younger then me, believed in Santa full on until my cousin told her at 10 and she came crying to me. I got mad a my cousin because it was fun to believe in Santa and I liked pretending to believe for my sister. My sister was not traumatized nor did she feel lied to by my parents or me. We still continued to play Santa at Christmas and leave him stuff etc. My sister carried on our family traditions incl Santa with her children. I have lots of wonderful memories of telling my sister stories about Santa, listening for reindeer and the excitement of trying to sleep so Santa would come. I always wished that I really did believe instead of pretending to believe and I felt a bit cheated by my mom for telling me the truth and for not helping me to keep that Christmas magic like I did for my sister.
I will be doing Santa with my son, I have been waiting a long time to have that true excitement and magic in our home. Lots of people believe in things that I do not surrounding religion, to me believing in Santa is on the same lines of believing in those things and it doesn't seem to far fetched or like lying to my child to help him believe in the jolly old elf

Oh I also just wanted to say that Santa fills the stockings for everyone and brings one gift (unwrapped) to the children (usually off their list when they are old enough to write a list)