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. If you are a parent of a younger teenager, you may or may not be a good judge of whether or not your kids can deal with it. The Potter's Hands Suggestion as used in self-hypnosis is a potent force that directs the subconscious mind toward the achievement of consciously chosen goals.