There is no option like that on the Keyboard option. How can I start it, I thought it was an inbuilt feature and reading other pages they have it. The iOS version is 7.1.2
This isn't a keyboard feature, it's an accessibility feature.
So go to Settings -> General -> Accessibility
Under vision you will see there is a button called Speak Selection, just press that and turn it on. When you select text you then have an option to let it read for you.
If this doesn't help, let us know, thanks. :)