I once heard there is an opinion in Rishonim (maybe Gaonim?) that says we count ones years of life according to the solar calender. So for example a boy would become bar mitzvah when he would turn 13 years old according to the solar years. Where is this opinion brought if at all?
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R' Gil Student cites the Ibn Ezra
Nevertheless, insofar as there are halachic implications, R' Student understood the Bar Mitzvah to follow the Hebrew Calandar (as is the custom and I do not believe there is sufficient reason to assume Ibn Ezra's general words should be understood to the implying otherwise).