See this tip to find out if your Mac can install Sierra from a system as old as 10.6: Not all 10.6.8 compatible Macs are compatible with 10.11 either. Apple has provided an update to 10.11 for these Macs while leaving 10.10 and 10.9 off the App Store as discussed here. Some Macs may not be able to take Sierra, but can take 10.11 and came with 10.6.8 or earlier. Any version of 10.7 can be upgraded in this sequence before installing newer operating systems if the hardware is supported: Sierra can be upgraded from 10.7.5 directly if all its associated updates have been installed. The only exception is that High Sierra needs a minimum of 10.8 installed, and will reformat SSD and and Flash drives to APFS rendering it invisible to Sierra and older systems. High Sierra is also still available as of September 30, 2018. Macs that shipped with Sierra can be updated directly to Mojave from the Mac App Store. Note, it no longer has 'X' in the official name.

The official release date of Sierra was September 20, 2016. This is a mirror of the original link on which is no longer being maintained.

Upgrade mac 10.7.5 to 10.9