https://en.wikipedia.org/wiki/IPhone

C-f for "Supported OS release" and there's a chart showing which devices support which OS. 12 was "sticky" in that it was the last one supporting the 5S and 6. 15 will be similarly sticky given the range of devices (6S/SE(2016)/7) that will be stuck with it and will probably get several more years of security updates as a result.