I went to ccd (bible study basically) so they told me when I was very little that he used to be a real person but died hundreds of years ago. So I never really believed in him. I think most kids figure it out before the age of 10. I know my aunt and uncle refused to tell my cousin about santa and he still believed into age 13 and said something at school and all of his friends made fun of him for believing in santa. I don't think I would let it go past 7 or 8 but that's me.