How does one determine one’s bar (or bas) mitzvah date if one is born in Adar of a ‘regular’ year, when the year where he (or she) will reach the age of being obligated in mitzvos is a year with two Adars?
The Gemara that deals with the question of whether Purim is to be celebrated in Adar I or Adar II states that even according to the opinion that Purim is celebrated in Adar II, the fourteenth and fifteenth of the first Adar are both forbidden to be days of fasting, and no eulogies are to be said on them. Tosafos understand that this opinion maintains that there is no obligation to feast and make merry on those days in Adar I according to this opinion; rather, one must only avoid overt signs of sadness. Yet this raises the obvious question of how one can differentiate between the two types of “celebrating” these days in Adar I! The Vilna Gaon (Shulchan Aruch Orach Chaim 568:7) suggests the following differentiation: The prohibition against fasting and eulogies on the fourteenth and fifteenth of Adar I is not based on the Megillah’s mandate that these days are to be days of partying and feasting. Rather, the prohibitions’ origins lie in that these days are included in what is known as ‘Megillas Taanis,’ —a chronicle of Rabbinic Holidays on which fasting and eulogies are forbidden because of the miracles that occurred on those days. These dates, the fourteenth and fifteenth of Adar are indeed listed in Megillas Taanis. The Gra then explains that any laws that are involved in celebrating Purim are observed, according to this opinion, during the second Adar. However, with regards to laws that are engendered by a calendar date, we indeed follow the date. Thus, any day that has the date of the fourteenth or fifteenth of ‘Adar,’ —whether it is Adar I or Adar II— is subject to the prohibitions on fasting and eulogies as per Megillas Taanis. Megillas Taanis deals with dates —not with Purim. If the date is in Adar, well then, Adar is Adar, and there are two of them! (The Gra points out that the Gemara in Megillah makes it clear that were it not for a special derashah from a passuk, Purim itself would be celebrated in both Adars!)
The Gra’s explanation was actually written to explain the law regarding yahrtzeits. The issue of setting the date of yahrzeit in a year with two Adars for someone who passed away in a year when there is only one month of Adar is the subject of a three-way dispute amongst the Poskim. The Shulchan Aruch rules that, in such a case, one commemorates the yahrzeit in Adar II (as is the case with deciding when to celebrate Purim). Rema maintains that one observes the yahrzeit in Adar I. The basis of this ruling is a Gemara in Nedarim, which states that when dating legal documents, one simply writes ‘Adar’ (with no additives) in Adar I, and ‘Adar II’ in the second Adar. The Gra, in addressing the Rema’s opinion, states that the terminology used when dating a document is not conclusive proof in these matters. There, the decisive factor is merely how people generally refer to the month, which has no inherent legal status. (Apparently, in Rema’s experience, people referred to Adar I as Adar, and to Adar II as Adar II.) The Gra thus goes on to say that, in his opinion, since the question of a yahrzeit deals with a date —the commemoration of the yahrzeit should be in both Adars. Since we dealing with a calendared date, and the date is Adar —Adar is Adar, even if there are two of them! Thus, the Gra holds that the fourteenth and fifteenth of both Adars are days when fasting and eulogizing are prohibited, as per Megillas Taanis. In contrast, feasting and partying are dependent on Purim —of which there is only one! (We then have the argument in the Gemara as to in which Adar we should celebrate Purim, and the conclusion is that it is the second one).
Here is another approach. The Yerushalmi in Masechess Megillah states that the primary Adar is indeed the second one. The first Adar is the ‘extra’ month, inserted, as we have learned, so that the lunar ‘year’ can catch up with the solar one. Therefore, writes the Yerushalmi, a lamb born in Adar of a regular year is not to be considered a year old yet (and thus disqualified from certain korbanos), if the following year has two Adars, until the second Adar —which is the only true Adar. Many point to this Yerushalmi to explain the ruling of the Rema that a child born in a regular year in Adar becomes bar mitzvah in Adar II.
Thus we can say that, if it is a question of age —based on the Yerushalmi the age is only reached on the date of birth, which regarding Adar is only the second Adar, for in the first Adar we are merely ‘catching up’ to the solar year.
Regarding the calendar date (e.g., a yahrzeit) —Shulchan Aruch rules that the commemoration should be in the second Adar, Rema rules that it is held in the first, and the Gra rules—both! When it comes to dating a document, which apparently is determined by the way people refer to the month, Adar would be the first Adar.
It is interesting, is it not, that one’s age is so directly determined by one’s birthday. Meaning that a boy who was born, say, 16 Adar I celebrates his birthday on 16 Adar (if his bar mitzvah year has only one Adar. That’s fine —but isn’t he at least as old as the child born, say, three weeks later, seven Adar Two, who becomes bar mitzvah nine days earlier, 13 years later on 7 Adar? How can it be that we make the first child wait another nine days —isn’t it a question of age? He is at least as old —and older— than the second child is! We see that age is, legally at least, a function of how many birthdays one has passed. That is a chiddush —think about it!