Tefillin
The custom of laying tefillin begins when a boy reaches bar mitzvah age. In some Orthodox circles, however, the custom is for the Bar Mitzvah boy to begin putting Tefillin on one to three months before his Bar Mitzvah. This way, by the time he is obligated in the commandment, he will already know how to fulfill it properly.
Read more about this topic: Bar And Bat Mitzvah