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 that 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 alck 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.
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?
Is a Tumtum/Androgynus obligated to wear Tefillin?
May a male who became a transvestite/transgender wear Tefillin?
He is obligated to wear Tefillin. It should be done in private.
Is a Shoteh/Insane to put on Tefillin?
A Cheresh and Shoteh are not obligated to wear Tefillin. However, they may choose to do so as long as they are able to maintain a clean body.
Is a deaf or mute obligated to put on Tefillin?
A Cheresh, which is a deaf-mute, is not obligated to wear Tefillin. However, they may choose to do so as long as they are able to maintain a clean body. However a deaf who is not a mute or a mute who is not deaf is obligated to wear Tefillin.
May a gentile wear Tefillin?
May one put on Tefillin to a person who is questionably Jewish [i.e. Mivtzaim]?
This depends on the reason of doubt. In general, if the person is unsure whether his mother is Jewish then one is not to put Tefillin on him, until his Jewishness is verified for certain. If one says his mother is Jewish, he may be believed regarding the Mitzvah of Tefillin, although regarding other matters, such as marriage he may need to provide proof of his Jewishness. In any case of doubt of a persons Jewishness one is to speak to a Rav who is expert in these matters to determine the severity of the doubt and whether one may put Tefillin on the person.
 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 Sefaradim: The custom of the Sefardim 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 Sefaradi communities have the custom that the boy does not begin to wear Tefillin until several years after his Bra 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; 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 ebcuase the Yasom has no one lese 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
 M”B 38/10
 Toras Chaim Sofer 38/3
 Piskeiy Teshuvos 38/4
 Olas Tamid 37; Shiyurei Kneses Hagedola 38; Beir Heiytiv 37/1; M”B 37/12
 Rambam Melachim 10/10
 Radbaz ibid
 The reason: As in general a person is assumed to be a gentile unless proven otherwise, as Kol Haporeish Merubo Poreish. [See Admur 329/2; Michaber Even Haezer 4/34]
Verfiying one’s Jewishness: One is considered questionably Jewish in any case in which he is unsure if his mother was Jewish, or if his proclamation of Jewishness requires backing, as stated in the cases below. The verification of one’s Jewishness can be done in several ways: 1) Checking the legal identification of the person or his mother, and seeing if it says Jewish. 2) Verfiying the Jewish names of the person and his family. 3) Knowledge of Yiddish, and acting like a Jew or knowedlge of Jewish customs. The person in question must fulfill at least two of the above three conditions to be considered a Jew, and only in a case that there is no other reason to cast doubt on his Jewishness. [opinion of Rav Moshe Feinstein] In today’s times, one also heavily relies on the opinion of an investigator who is trained and experienced in verifying whether the person’s claim is true or not. [So is done today in all Batei Dinim] Nevertheless, the above applies regaring allowing the person in question to marry a Jew. However regarding Tefillin, seemingly one may rely on a more lenient stance, so long the person acts like a Jew and says he is Jewish. [See Michaber 268/10 that we are only stringent to require proof in order to marry the person]
 If one says he is Jewish because his mother is Jewish is he to be believed? If a person says he is Jewish and acts like a Jew then in general he is believed to be Jewish, as people contain a Chezkas Kashrus, and majority of people who say they are Jewish are Jewish. [Michaber Even Haezer 2/2; Rashal Kesubos 2/40; Rabbeinu Tam in Yevamos 46b; Rashba; Ramban] Some Poskim rule that this applies even if the person came from a different country. [Shach 268/21; Bach Yoreh Deah 268; Halef Lecha Shlomo Even Haezer 15] Other Poskim however rule that if he came from a different country then he must prove his Jewishness, even if he acts like a Jew. [Beir Heiytiv Even Haezer 2/4 in name of Maharit 1/149 and Beis Hillel, and so rules Michaber Y.D. 268/10 regarding a gentile who says he converted; See however Aruch Hashulchan 2/13 that this only applies to an individual, while a family does not have to prove their Jewishness if they act like a Jew, as a family maintains a Chazaka.] Practically, we no longer rely on the above Chazaka regarding Jews who come from areas where intermarriage is common, such as the USSR. This especially applies if there is a reason to place doubt into the persons Jewishness. Nevertheless, regarding Tefillin, seemingly one may rely on the Chazaka if the person acts like a Jew and says he is Jewish, and so is the custom. [See Michaber 268/10 that we are only stringent to require proof in order to marry the person]
If one says he is Jewish because he converted, is he to be believed? If the person was known as a gentile and now claims to have converted, then he is not believed to be allowed to marry a Jew unless he supplies witnesses or re-converts. This applies even if he acts like a Jew. [Michaber Y.D. 268/10; Yevamos 46b] If however the person was held to be Jewish and says that in truth he converted, then he is believed without witnesses. [Michaber ibid] Now although some Poskim argue that even such a person must bring witnesses [Rambam in Michaber ibid] nevertheless the custom is like the Michaber. [Shach 268/21 in name of Bach]