Question: Our son was born in Adar and he turns thirteen during a leap-year. I was always under the assumption that he turns Bar Mitzva in Adar Sheni, but we were told to observe a relative’s yartzheit in Adar Rishon. Are they not the same?
Answer: There is a machlokes among the Rishonim as to whether ‘Adar’ during a leap-year generally refers to Adar Rishon or Adar Sheni. Rambam (Nedarim 10:6) writes that ‘Adar’ refers to Adar Sheni (See Kesef Mishna), while the Ran (Nedarim 63b) and Rosh (Nedarim 63a) write that it refers to Adar Rishon.
When one’s relative passes away during one of the Adar’s in a leap year, the yahrzeit is observed during that same month in a leap year; either Adar Rishon or Adar Sheni. However, when one’s relative passes away during Adar in a non-leap year, there is a machlokes as to which month the yahrzeit should be observed in. The Shulchan Aruch (OC 568:7) writes that it should be observed in Adar Sheni. The Kaf Hachaim (OC 568:76) notes that this is the main Sefardi practice (See Bach YD 2201:12; Shach YD 220:16). Nonetheless, the Shulchan Aruch (CM 43:28) writes that in legal documents, it is presumed to be Adar Rishon.
The Rema (YD 402:12; OC 427:1; 568:7 quoting the Terumas Hadeshen 294; Mahari Mintz 9) disagrees, writing that it should be observed in Adar Rishon. (See Mateh Moshe 766; Chochmas Adam 171:11; Gesher Hachaim 32:10), though notes (OC 568:7) that some observe both (See Mishna Berura 568:42; Chassam Sofer OC 163; Tzitz Eliezer 22:39).
The Rema (OC 55:10) writes that a boy born in Adar in a non-leap year must wait until Adar Sheni to be considered Bar Mitzva as only then is he thirteen halachic years old.
In conclusion, a boy born in Adar in a regular year must wait until Adar Sheni to celebrate his Bar Mitzva. Sefardim typically observe yartzheits of those who pass away in Adar in a regular year, in Adar Sheni, while Ashkenazim typically do so in Adar Rishon.