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?
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).