10.11 Installation on old Macs
Unless your models are Early for the years, they should be supported.


http://www.apple.com/osx/how-to-upgrade/#hardware-requirements


Apple requires you to have 10.6.8 or newer running since that's where the Mac App Store runs. If you have access to any Mac running 10.6.8 or newer, you could sign in to the App Store and download the 10.11 installer. It has a script to make a bootable USB installer - one shot command:


https://support.apple.com/en-us/HT201372


If you run into issues, this thread has several other methods to work through fixing your installer.


How do I make a bootable USB flash drive a Mac?


If you're not seeing the USB drive or network boot, perhaps resetting the NVRAM might help with that.