Replacing El Capitan with Mavericks I'm trying to install Mavericks from a bootable USB. 
I'm using this guide; http://macpaw.com/how-to/clean-install-os-x-mavericks-on-my-mac

I delete El Capitan in disk utility, but there is no 'Install OS X', merely 'Reinstall OS X', which asks me to reinstall El Capitan. 

Where is the option to roll back to previous operating systems?

EDIT: This is on an early 2015 model Apple Air.
You have not written what your MAC, ie, it is unclear whether the installation it is possible Mavericks.

Minimum Supported Operating System for your computer can be viewed using
Mactracker or MachineProfile

Additionally:


  Back in February, Apple's Worldwide Developer Certificate, designed to
  verify third-party apps and services, expired, requiring Apple to
  issue a new certificate for developers to use. OS X installation files
  downloaded from the Mac App Store before February 14, 2016 and stored
  on a computer or USB drive are no longer functional. This includes
  installation files for OS X El Capitan and older versions of OS X like
  Mavericks and Mountain Lion. Users who keep OS X installers on hand or
  have created USB install disks in case of emergencies should replace
  their files with new versions by re-downloading them through the Mac
  App Store using purchased history.
  
  Users who can't replace an expired certificate can still use older
  installers by changing the dates on their machines.
  
  
  In the OS X Installer, choose Utilities > Terminal.
  Enter date 0201010116 and press Return.
  Quit Terminal and continue the install.
  


OS X Installers Downloaded Prior to February 14 No Longer Work After Certificate Update