With each new OS version, Apple eliminates support for older hardware, even if it's still capable of running the new OS.
Planned obsolescence or just a push for you to make an upgrade? It doesn't matter, actually, as there are still ways to use the newest OS on older Apple computers, such as:
Early-2008 (or newer) Mac Pro, iMac, or MacBook Pro Late-2009 (or newer) MacBook Air or unibody MacBook Early-2009 (or newer) Mac Mini or white MacBook
What do you need?
1. An 8Gb or bigger USB memory stick 2. MacOS Sierra Patcher for Mac - it will create a custom USB installation 3. MacOS Sierra installation App 4. AUSEnabler - it will help you get future software updates on your installation.
For known issues, read here.
Steps:
1. Go to Spotlight - System Information - Hardware Overview - Model Identifier and note the value. In my case was MacBook5,1
2. You need Install macOS Sierra.app in your Applications Folder.
You can get this from a newer (supported system), from a friend or if you have a supported computer.
3. Run MacOS Sierra Patcher for Mac and, using the 8Gb+ USB memory stick, create a custom bootable installation kit.
4. When the creation of the tool is finished, reboot the computer, keep ALT/OPT key pressed, and select the USB device on boot.
5. Install MacOS as usual and wait for the process to finish. It will restart, but will not work, so you need to repeat the step 4 and boot from the USB drive, to run post installation tweaks.
In the first interface after boot, select from the top menu: Utilities - MacOS Post Install
Now, you must select the correct Model Identifier for your hardware (see Step 1) and apply the suggested tweaks and reboot!
6. Use AUSEnabler to get software updates.
That's it!