When Snow Leopard 10.6 came out in August 2009, it only supported Intel Macs, which were at earliest from January 2006, about three and a half years earlier. Typical Mac OS X releases up to El Capitan (september 2015) supported 10 years of Macintosh computers. Sierra/High Sierra still do, essentially.
The support for older machines depends on multiple factors, like the multi-year transition from 32 bit to 64 bit for the OS, including 32 bit EFI, 64 bit EFI, etc. Some older machines were not supported by newer releases (not talking about security patches) for longer than five years, IIRC. That's a long period of time, but still quite short when compared to Windows support of older machines during that time (this is not a great comparison because of differences in annoyances and quality in the OSes, but more about the companies' work).
