I remember having a conversation with my wife when my oldest was about six asking her how long did she think he would believe in Santa. She was sure he had already figured it out, but was playing along for the extra presents "From Santa."
When my youngest got to be about six she stubbornly announced "Santa isn't real!" because she is that kind of kid. My middle daughter was ready to murder her because she thought that would mean fewer presents under the tree.
But I know for a fact that Santa is real. I'm his favorite photographer and work with him at the local mall taking pictures with the kids around this time of year.