"Bar/Bat Mitzvah gifts
As with weddings, it is common to give the Bar or Bat Mitzvah celebrant a gift to commemorate the occasion. Traditionally, common gifts included books with religious or educational value, religious items, writing implements, savings bonds (to be used for the child’s college education) or gift certificates. Gifts of cash are commonplace in recent times. As with charity and all other gifts it has become common to give in multiples of 18: the gematria, or numerical equivalence of the Hebrew word for "life", ("chai") is the number 18. Monetary gifts in multiples of 18 are considered to be particularly auspicious and have become very common for Bar/Bat Mitzvahs. Many Bar/Bat Mitzvah also receive their first tallit from their parents to be used for the occasion."
(Wikipedia - Bar and Bat Mitzvah)
http://en.wikipedia.org/wiki/Bar_Mitzvah_/_Bat_Mitzvah