Wearing Tefillin before the Bar Mitzvah: 
A. When to begin wearing:
The dispute: It is disputed as to when a father becomes obligated to buy Tefillin for his son, and educate him in the Mitzvah of wearing Tefillin. Some Poskim rule a father is obligated to buy Tefillin for his son, to educate him in Mitzvos, as soon as his son is mature enough to guard the Tefillin with purity. This means that the child is mature enough to refrain from flatulence while wearing them, and to refrain from sleeping in them, and refrain from entering with them into a bathroom, even to urinate. [This is approximately the age of 9 or 10 years old.] However there is an opinion who says that this level of maturity only applies to a child that is over 13 years of age but has not yet grown two hairs, [in which case he is to be educated in Tefillin despite not being Biblically obligated due to his lack of two hairs]. However, a child that is under 13 years of age is not at all to be educated to wear Tefillin, as he does not yet have the knowledge to guard it.
The final ruling: Practically, the old custom of Ashkenazi Jewry is not to educate the child to wear Tefillin until he becomes Bar Mitzvah. Nevertheless, today the custom is to educate the child to wear Tefillin 2-3 months prior to his Bar Mitzvah. [The Chabad custom is for the child to wear the Tefillin two months prior to his Bar Mitzvah. At first however the child is to start wearing the Tefillin without a blessing. After the passing of a few weeks, when the child now knows how to wear the Tefillin properly, a blessing may be recited. The child is to remain wearing the Tefillin during the entire Shacharis prayer.]
When is the child to be educated in Rabbeinu Tam, before or after his Bar Mitzvah?
Today the custom is to educate the child to wear Rabbeinu Tam from the same time as when he begins to wear Rashi Tefillin, which is two months before the Bar Mitzvah.
If the child will be Bar Mitzvah within two months from after Sukkos or Pesach is he to begin wearing the Tefillin before two months?
If Chol Hamoed will fall within the two months the child may begin to wear the Tefillin a week earlier.
Shavuos: Accordingly, the same would apply if two days of Shavuos falls on a weekday within the two months, the child may begin to wear Tefillin two days earlier.
Is a Yasom to begin wearing Tefillin one year before his Bar Mitzvah?
B. The Seder Hanachas Tefillin for a Bar Mitzvah boy:
Practicing at night: It is permitted for the pre-Bar Mitzvah to practice wearing the Tefillin at night, to see that it fits etc, so long as he has intent to do so not in order to fulfill the Mitzvah.
Shabbos Clothing: It is customary for the pre-Bar Mitzvah boy to wear Shabbos clothing on the day of the Hanachas Tefillin. Some fathers are also accustomed to wear Shabbos clothing on this day.
Mikveh: The pre-Bar Mitzvah boy is to immerse in a Mikveh on the day of his Hanachas Tefillin.
Perek 41 in Tanya: It is customary to recite the beginning of chapter 41 in Tanya prior to Davening and wearing Tefillin. Prior to the Bar Mitzvah, when the child is training in putting on the Tefillin, he should commit the above section of the chapter to memory.
The intentions/Kavana to have when putting on Tefillin: One is to teach the child the correct intents to have upon putting on the Tefillin, as brought in Chapter 2 Halacha 3!
Fixing the knot of the Daled of the Shel Rosh: If the straps of the Shel Rosh need to be adjusted so they fit his head properly, one must make sure to say Lisheim Kedushas Tefillin beforehand. See Halacha 8.
Wearing a hat: Seemingly, pre-Bar Mitzvah children are not yet to begin wearing a hat for Davening being that it may confuse others to think that they are already Bar Mitzvah, and have them unknowingly join him for a Minyan or various areas of Davening that require a Minyan. Nevertheless, some are accustomed to train the child to wear a hat already starting from his Hanachas Tefillin. Each community should follow their custom, and there is no set Chabad custom in this matter.
Chazaras Dach-Lechaim: It is customary of Chassidim that on the day of the Hanachas Tefillin, after Shacharis, the pre-Bar Mitzvah boy gathers with family, friends, and other adults, and reviews from memory the first chapter in the Mamar of the Rebbe Rayatz titled Isa Bemidrash Tehillim. This is preceded by the Hachanah Niggun for a Mamar, and concluded with the after Mamar Niggun. After the Mamar is recited Lechaim and refreshments are distributed, in which people bless the Chassan Bar Mitzvah Mitzvah and his father with a Mazal Tov and that his son grow up to be a Chayal, Chassid, Yireh Shamayim and Lamdan. It is customary for the father and/or child to say a few words of Torah at this time. Regarding drinking Lechaim while wearing Tefillin, this matter is under debate amongst the Poskim, although a small amount of Lechaim is allowed to according to the mainstream approach. One is to make sure not to eat more than 55 grams of Mezonos while wearing the Tefillin. See Our corresponding book “The laws of Tefillin” Chapter 5 Halacha 3.
C. Teaching the child the laws of Tefillin:
It is the father’s responsibility to teach his son the laws of Tefillin prior to his Bar Mitzvah.
D. Should one recite the blessing of Shehechiyanu upon putting on Tefillin for the first time?
Some Poskim rule the blessing of Shehechiyanu is to be recited upon performing a Mitzvah for its first time, and hence a Bar Mitzvah boy is to say the blessing of Shehechiyanu upon wearing the Tefillin for the first time on the day of his Bar Mitzvah. However, other Poskim argue that one never says the blessing of Shehechiyanu upon performing a new Mitzvah and therefore no blessing is to be said upon putting on Tefillin for the first time. Practically, those who begin putting on Tefillin 2-3 months prior to the Bar Mitzvah are not to recite the blessing of Shehechiyanu, neither upon putting it on for the first time before the Bar Mitzvah, nor on the day of the Bar Mitzvah. However, those who are accustomed to put it on for the first time only on the day of the Bar Mitzvah, are to say Shehechiyanu on a new fruit or new clothing, and have in mind to include the Tefillin. Furthermore, it is proper by every birthday for the person to recite Shehechiyanu on a new fruit or garment, and certainly this applies to the day of the Bar Mitzvah.
 Admur 37:3
 1st opinion in Admur ibid; Michaber 37:3; This is approximately the age of 9 or 10 years old-Kaf Hachaim 37:14
 Kaf Hachaim 37:14
 2nd opinion in Admur ibid; Rama 37:3; Baal Haitur Tefillin 7
 Admur ibid; Rama ibid “And one is not to swerve from this custom”; Shulchan Hatahor 37:4 based on tradition from Baal Shem Tov and based on Kabbalah; Os Chaim Veshalom 37:5; Darkei Chaim Veshalom 79; See Shulchan Menachem 1:105 for a letter in which the Rebbe defends this practice, even though it is not the Chabad custom.
Custom of Sephardim: The custom of the Sephardim follows the 1st opinion mentioned above [the opinion of the Michaber] and they hence put on Tefillin with a blessing beginning from two years before the Bar Mitzvah. [Or Letziyon 2:44-47; Yabia Omer 6:3; See Kaf Hachaim 37:14] Some Sephardi communities have the custom that the boy does not begin to wear Tefillin until several years after his Bar Mitzvah. This custom is to be completely negated. [Letter of Rebbe in Shulchan Menachem 1:105]
Other customs: Some rule one is to begin wearing Tefillin at age 12. [Bach 37; See also P”M 37 A”A end] Some are accustomed to wear the Tefillin one month prior to the Bar Mitzvah. [Aruch Hashulchan 37:4 that so was the custom in Poland and Lithuania; See Igros Kodesh 14:463]
Custom of Rabbeim: The Rebbe Rayatz began wearing Tefillin in private with a blessing at age 11. [Igros Kodesh 3:136 [printed in Shulchan Menachem 1:36]
 Admur ibid; M”A 37:4; M”B 37:12; See Shelah in Mitzvas Tefillin p. 32 that today that Tefillin is only worn for prayer the ruling of the Baal Haittur does not apply
 Hayom Yom 2nd Av based on the public directive of Rebbe Rayatz; Igros Kodesh Rayatz 7:24; Sefer Haminhagim p. 4; Igros Kodesh 11:76; 16:305; 17:61; 14:463 [printed in Shulchan Menachem 1:104-109]
 Hayom Yom 2nd Av based on the public directive of Rebbe Rayatz; Sefer Haminhagim p. 4; See Igros Kodesh 14:63; 11:76, [printed in Shulchan Menachem 1:109]
 Igros Kodesh 18:266 [printed in Shulchan Menachem 1:109]
 Toras Menachem 1989 4:142 [printed in Shulchan Menachem 1:107]
 Likkutei Sichos 21:357; Igros Kodesh 23:89; Shulchan Menachem 1:36; Rav Groner related to me that the Rebbe replied this answer to many people.
 See Likkutei Dibburim 1:214 that the Rebbe Rayatz began to wear Tefillin on the 11th of Iyar [and not the 12th of Iyar] seemingly because Shavuos fell that year on Shabbos-Sunday and hence one day was lost.
 Custom is mentioned in Aruch Hashulchan 37:4; Os Chaim Veshalom 37:5; Mikdash Meat 37:3; Taamei Haminhagim Tefillin 23; Likkutei Maharich; Pinos Habayis 85; Igros Kodesh 3:136 [printed in Shulchan Menachem 1:36]
The reason: Some suggest the reason for this is in order to elevate the soul of his parent. [Taamei Haminhagim ibid; Pinos Habayios ibid, mentioned in Igros Kodesh ibid] Alternatively, the reason is because the Yasom has no one else to educate him to wear it and hence we do it much time in advance. [ibid]
 Aruch Hashulchan ibid; Os Chaim Veshalom ibid; Darkei Chaim Veshalom 79; Igros Kodesh ibid
 See Nitei Gavriel Bar Mitzvah chapter 39
 Shevet Hakehasi 6:15; Piskeiy Teshuvos 30:2 footnote 17; Nitei Gavriel 39:16
 Nitei Gavriel 39:2
 Nitei Gavriel 39:3
 Igros Kodesh 11:340; 14:533; Shevach Yakar 37:7, p. 100
 See Shevach Yakar p. 253
 Printed in Sefer Hamamrim 5708 p. 271
 M”B 37:9
 See Piskeiy Teshuvos 22:2; Likkutei Sichos 11:289; Hisvadyus 5742 2:1112; 5748 2:403; Shevach Yakar p. 100
 Taz 22:3; Olas Tamid 22:1; Chasam Sofer 55; Rama Y.D. 28:2 regarding Kisuiy Hadam; Tevuos Shur 28, brought in P”M Y.D. 28 S.D. 5; Rambam Brachos 11; Rokeiach 371 in name of Rivak Mishepira; Opinion in M”B 22:1 regarding Tzitzis; Biur Halacha 22:1 “Kanah” in name of Gr”a
 Shach Y.D. 28:5; Tosafus Sukkos 46a; Ran Chulin 88; Rashba 126; Peri Chadash 28; Tzevi Latzadik 28; M”A 22:1; Mamar Mordechai 22:1; Machazik Bracha 22:2 that so is opinion of Michaber 22:1 and that so is the custom not to recite Shehechiyanu on Tefillin; Chayeh Adam 62:10; M”B 22:2; Kaf Hachaim 21:2 in name of many Poskim; Maharik Shoresh 128 regarding wedding; So rule regarding Tzitzis: Michaber 22:1; Admur 22:1; Baal Haittur; Mahariy Abuhav, brought in Biur Halacha ibid
 The reason: As the sages only instituted to recite the blessing of Shehechiyanu by a Mitzvah that contains joy. [Shach ibid in name of Tosafus ibid] Alternatively, the reason is because it does not come from time to time. [Shach ibid in name of Ran ibid] Alternatively, by Tefillin the reason is because one is meant to put it on already as a child, when one cannot yet say Shehechiyanu due to the lack of obligation, and therefore the sages never instituted this blessing to be said by Tefillin, even when worn for the first time by an adult. [Tzevi Latzadik ibid] Alternatively, the reason is because the Simcha by a Bar Mitzvah only comes from the future knowledge of the successful indoctrination of the child to become a servant of G-d, and is not readily apparent. [Rebbe ibid in Hisvadyus 5748]
 Tevuas Shur Y.D. 28:4; Ashel Avraham Butchach 37; Tzevi Letzadik ibid; Likkutei Sichos 11:289; Hisvadyus 5742 2:1112; Shevach Yakar p. 100; Piskeiy Teshuvos ibid footnote 10
 Biur Halacha 22:1 “Kanah”; Sefer Hachaim; Beis Shearim 30; Beis Hayotzer 4; Sdei Chemed Mareches Brachos 20; Ben Ish Chaiy Riei 17; Kaf Hachaim 22:2; 225:11; Ginzei Yosef 4; Likkutei Teshuvos Minchas Yitzchak 18; Minchas Shlomo 2:4; Piskeiy Teshuvos ibid and the many Poskim listed in footnote 7; See Darkei Teshuvah 28; Teshuvah Meahavah 5; Lehoros Nasan 2:3; Imrei Yosher 5; Tzitz Eliezer 13:24; Yechaveh Daas 2:31; Mishneh Halachos 6:6
 Hisvadyus 1988 vol. 3 p. 152  letter 10