For most purposes, MacTeX is the Mac distribution of TeX Live. As with all other platforms, groups of volunteers compile new binaries for TeX Live every year, and the MacTeX group is responsible for compiling the binaries for the Mac. They also choose to distribute some other useful GUI program along with the TeX Live distribution including TeX Live Utility, BibDesk, and TeXShop and TeXWorks. These programs plus the TeX Live distribution constitute what is called MacTeX.
This year, because of increasing difficulties in providing compiled binaries for eight different versions of the OS (including versions that still ran on PPC machines), the group decided to support only versions of the MacOS for which Apple is still releasing security updates. At time of writing, this means that they will support Yosemite (10.10), El Capitan (10.11), and Sierra (10.12) and the upcoming High Sierra (10.13).
So there is no MacTeX compiled TeX Live that you can install on such an old machine, I'm afraid.
However, it seems that binaries for older versions of the Mac are still being compiled, and therefore it does seem to be possible to to install a current version of TeX Live using the TeX Live net installer.
The basic directions for the Mac are found here:
However, since so few Mac users install MacTeX this way, there seems to be very little other information about how this works. Having never used the net installer, I don't know how it goes about identifying the correct binary for your machine. There may also be issues with getting an up-to-date Ghostscript as well.
@egreg confirms here that he has managed to get TL2017 binaries running on OS 10.7 (Lion) in a preliminary test.