Mac OS Sierra 10.12 was released on September 20, 2016.
Mac OS High Sierra 10.13 was released on September 25, 2017.
Mac OS Mojave 10.14 was released on September 24, 2018
Only 10.12 and 10.13 operating systems have the same minimum system requirements.
10.14 requirements are the end of this tip.
A few Macs which came with Mac OS X 10.6 Snow Leopard can install Sierra.
10.6.1 shipped new with the MacBook Late 2009 and the iMac Late 2009 (can use 10.6.3 retail for install).
10.6.3 shipped new with MacBook Pro Mid 2010, and 10.6.4 shipped new with the MacBook Air Late 2010, Mac Mini mid 2010, and the Mac Pro Mid 2010 (all must use the AppleCare provided 10.6 installer disc for the model if you are missing it).
System requirements for Sierra require the App Store of Mac OS X 10.7.5 to install Sierra. All the Macs which are compatible listed above have firmware updates available that offer the internet restore functionality with installing 10.7 or later.
Check Apple support website for relevant firmware:
So once you have determined if your Mac can install Sierra, be sure to read the compatibility guidelines before installing 10.7 or later, and then 10.7.5 to make the update to Sierra.
10.8 can take you up to High Sierra for the same Macs that can go to Sierra from 10.7.
Some older Macs that came with 10.4 and 10.5 can install up to 10.11, but not Sierra.
Only some Mac Pros that shipped with 10.6 can be updated to 10.11, and then 10.14 Mojave.