Along with a new suit and all the other accoutrements for a barmitzvah boy is a pair of tefillin: two small black boxes holding parchments with biblical verses. According to halachah, the barmitzvah boy is expected to wear these on his left arm near his heart and on his upper forehead every weekday morning for the rest of his life as a reminder of his relationship to God.
Tefillin is a cherished mitzvah. Many women (who are not commanded to put on tefillin) have voluntarily taken on this commandment to feel the closeness to God that it offers.
Although a boy is not obligated to lay tefillin until he is 13, the custom is to start two months before the barmitzvah, without reciting the blessing.
Chinuch (education) is why we teach even small children to perform age-appropriate mitzvot. We wait until just before the barmitzvah for tefillin because we want the child to understand and treat the mitzvah with the respect it demands.