Why do I get "Warning: You are using macOS 10.9" from Homebrew when I have 10.9.5? I have just reinstalled OS X on my Macbook, I'm trying to get homebrew to install subversion, but I encounter the following error:

"Warning: You are using macOS 10.9.
We (and Apple) do not provide support for this old version.
YOu will encounter build failures with some formulae."
OS X 10.9 (Mavericks) is almost 5 years old, and has been unsupported since September 2016.  

You'll likely want to upgrade to macOS Mojave.