Getting back to the halachic issue of the double-Adar, the MG”A paskened that a child born in Adar I of a leap year does not become bar mitzvah until Adar II if his bar mitzvah also occurs on a leap year. However, R’ Akiva Eiger and others disagree. If a child is born in a regular Adar, then his birthday is celebrated in the “real” Adar, Adar II of a leap year. But a child born in Adar I of a leap year was not born during the month of Adar! The child was really born in the month of “tosefes” Adar. Therefore, he does not need to wait until “real” Adar to become bar mitvzah, but he becomes bar mitzvah once that same month of "tosefes" is reached.
(I happen to have a nephew whose bar mitzvah is upcoming in a few weeks during Adar II. If I am not mistaken, 1995 was a leap year as well, but I have no recollection if he was born in Adar I or Adar II.)
What remains a bit confusing to me is how to deal with the proof of the MG”A. The Yerushalmi in Kesubos does say that a three year old girl with an Adar birthday who is niveles during Adar will retroactively not be considered be’ulah if the year is made a leap year. The assumption seems to be that the girl’s birthday is postponed until Adar II. Why doesn’t the Yerushalmi in Megillah take this case as proof that the “real” Adar is Adar II? Is there anything gained by using the need for a korban less than a year old as a test for the “real” Adar vs using this well known case? The Ridba”z asks this questions and offers an answer, but after re-reading it a couple of times I still can’t make heads or tails of what he is saying. I will just note that interestingly the GR”A (O.C. 57) cites the Kesubos case as the source for the din in S.A. and does not cite the Yerushalmi in Megillah.
While not directly relevant to this topic, I have to mention a fantastic chakira which my wife thought of, which goes to show you what can happen after years of living with someone who tries to brainwash you into thinking in "lomdish" categories. Yesh lackhor: is Feb 29 of a leap year the last day in February, or the day after Feb 28? Nafka minah: if you are born on Feb 29 of a leap year, when do you celebrate your birthday? If Feb 29 is the last day in Feb, then on a non-leap year your birthday would be on Feb 28. But if Feb 29 is the day after Feb 28, then on a non-leap year your birthday would be March 1. I think this must be a discussion in achronim somewhere : )