Can't make postgresql load at startup in Mac OS
Use -w option with launchctl.
$ launchctl load -w ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist -w Overrides the Disabled key and sets it to false.
The new way as brew info postgresql says:
brew services start postgresql
It might also be helpful to use the command brew info postgresql for suggestions about how to set up your database and move the plist file into ~/Library/LaunchAgents