When kids stop believing in Santa Claus depends on the kid.
Some kids continue to believe in Santa Clause until they are past 12 years old while some kids may stop believing in Santa Claus by age 4 or 6 years old.
Me personally I stopped believing in Santa Claus at age 6 when I finally realized it was impossible for a man to fly around to every town and place in one single night.
I also caught my parents putting presents out and my dad was dressed in a Santa Suit.
So after that I stopped believing that Santa was real and was mad that my parents lied to me about Santa Claus.