What does my dream mean(may be scary to young people)?

The dream is about your feelings concerning something that your cousin is going through at this time. The coffin suggests an issue that your cousin needs to "lay to rest" something that is an emotional burden on his life that he needs to get over and move on. That the coffin containing this burden is following him around suggests that the issue is overshadowing his life and affecting his relationships. That he seems to be unable to move on from this issue, and that it is affecting his relationships also seems to be indicated by the coffin saying go away. The imagery suggests that your cousin won't share what it is that is bothering him, or talk to anyone about it, and this seems to be indicated by the green color of the coffin, he is keeping his true thoughts bottled up, and that is part of the reason this burden can't be laid to rest. The demons suggest inner demons. The imagery suggests a time of transition in your cousin's life though. Ten is a number of closure and completions, and the grim reaper also is a symbol for closure on a life situation, so I would look at the ten reapers as an indication that your cousin may be working through it. He may prefer to keep things to himself to work things out, and while this may cause concern on the part of the family around him, but whatever the burden is it is ultimately up to your cousin to work through it and lay it to rest.