A more direct answer to your question, as Diesel Fuel ages:
1. Water can get into the tank from condensation
2. The lighter hydrocarbons can evaporate and make starting harder and gum up the works.
3. As mentioned above, bugs can grow in Diesel Fuel. If growth is prevent, the fuel will be cloudy.
On a final note, all diesel fuel is really old. Like dinosaur old, lol. Had to through that in.