Any person of average intelligence and the ability to focus and concentrate can experience structured hypnosis.  The better one is able to concentrate, focus, and engage in the process, the easier it is for him or her to enter into a hypnotic state.

No one can be hypnotized if they do not want to be hypnotized.  Your natural psychological defenses prevent this from happening.  This refers to structured hypnosis as opposed to environmental hypnosis.