Well, if you study what makes some people and speech persuasive, you will not be easy to manipulate any more. Perhaps it will also enable you to manipulate those without such knowledge. Like an arms-race situation

Dont forget that some people are excellent manipulators without having to read about it in a book, its like natural talent for them

Thats why we protect minors - they are so inexperienced, so an adult could easilly manipulate them into sex, crime, anything.