When determining your Mac model, see this tip to find its age: Server versions of Mac OS X may have different limitations, and the people visiting the appropriate Server forum may be able to answer your questions better about Mac OS X Server. Note this tip, and the series of tips from 10.2 (10.2, and 10.3 not mentioned in above links as people must have 10.4.4 or later on an Intel Mac to get to 10.6) to 10.11 I've written here all refer to Mac OS X Client. A few Macs which came with 10.6 can install Sierra through Mojave. Mac OS X Sierra was released on September 20, 2016.
Tips on 10.4 Tiger, 10.5 Leopard(last compatible operating system for G4 867 and above, and G5s), 10.6 Snow Leopard ( end of the line for PowerPC applications), 10.7 Lion, 10.8 Mountain Lion, 10.9 Mavericks, 10.10 Yosemite, and 10.11 El Capitan to 10.14 Mojave.
