Yes, I have. It was my friend's younger brother.
2 I think nervousness or shyness may cause it. Sometimes when people are embarrassed or confused they might start to stutter as well. Some other people may stutter because they are scared.
3 Maybe people who stutter do some special exercises which help them to speak more easily. If stuttering is caused by some disease, maybe doctors give their patients medicine.
4 People who stutter must become more confident and more relaxed. Also I read that they are advised to speak slowlier and this might help them not to stutter.