Not thinking scripturally, but looking at the non-Jewish world as it is today, the majority of nations have adopted-- at least civilly-- the christian calendar which is based on both in ways. The Christian year is a solar year, eg, how long it takes for the earth to complete one full revolution around the sun, and return to the same place in its orbit, however, it is by lunar divisions of time (one "lunation"-- full length of time required for the moon to go through all phases and return back to the same, such as new moon to the next new moon)-- which vary at or around 28 days, that gives the idea of christian months. Lunations are not all the same because the moon's orbit around the earth varies significantly in the length and "oval-ness" of its path, in a fixed cycle-- but on average are about 28 days each. Of course, the christian months are further confused through the adoption of the Roman months of July and August, placed into the middle of the year, which are the reason the fall months are incorrect for their English (or, better yet, Latin), prefixes-- (eg, September, Sept- means 7, but it is 9, October, Oct- means 8, etc., all the way down to December with Dec- means 10 but it is 12.). Coupling the addition of the new Roman months with a fixed number of days in each month that are unequal, has, in a way, rendered the months in the Christian calendar completely irrelevant to the moon. Worth noting is, 28 days*13=364, so you could have 13 months and one day, to make up the 365 days of the solar year. Now, none of that would have to do with jewish law, but to the point that "the nations of the world," including completely foreign lands with traditionally other cutlures, such as far east Asia, for example-- all have adopted, at least civilly, the christian calendar...This would be the calendar that, reading the question you asked, concerning today, would be considered. It is based on the sun, but in a way is based on the moon...at least in theory.
So, measuring everything, including birthdays, and therefore birthdays concerning maturity or culpability, would be based thereon.
Presenting what is truly a non-answer, it falls to anyone to judge whether the system presently worldwide-adopted is fully solar or fully lunar, it is neither completely and it is both.
