Rabbi Shimon Eider Zatzal brings in his Sefer that the Minhag is to put on Tefilin from one to three months before the Bar Mitzva.
To the best of knowledge there are three different schools of thought as to when one should begin putting on Tefilin. On the day of the Bar Mitzva, one month prior, or three months prior. What is the reasoning and source for these different Minhagim?