If you are understanding at least most of what you are reading here, you are smart enough to do well with self-hypnosis. Concentration does have a bearing on the various levels of hypnosis. Better concentration means better and quicker hypnotic states. But if your concentration is poor, don't let that stop you. Self-hypnosis practice improves concentration.
Age also has a bearing on hypnotizability in some very general ways.
The very young and the very old do not respond well, sometimes not at all. Adolescents sometimes do well with self-hypnosis. It depends on what their glands are doing to them at the moment. It is probably a good idea to avoid teaching self-hypnosis to adolescents, especially those who are younger, because they are almost always interested in it for all the wrong reasons. If you are a teenager reading this, you are obviously an exception.