Bom-Bom-Bootz!: You can begin playing this with your child as soon as the soft spot in the skull, just above the forehead closes. Do not indulge in this game after your child reaches the age of two. There are reliable records of parents having suffered serious concussions from playing Bom-Bom-Bootz with overage youngsters.
--Stan Berenstain, Jan Berenstain - 1984, The Berenstains' Baby Book, p. 112