As a general rule - no, not everybody knows when they will die. However, some people are blessed with that knowledge. It's a very individual thing.  
 
Why the Lord blesses some people with that knowledge and some don't - I don't think we know the answer. 
 
But let me emphasize as mormons we absolutely believe in life after death and that all people on the earth will die and be resurrected. It's a great comfort to know that this life actually does have a purpose. A great place to go to gain some basis understanding about the purpose of this life can be found here: The Purpose of Life