At about 2 weeks in age is when colic usually appears. It doesn't necessarily happen in all babies, but is very common. In most babies, the crying usually starts the same time everyday and lasts for a set amount of time each time. Say for one child, the crying may begin at 7 each evening and last for 2 or 3 hours. The reasons for why colic occurs vary.
Gas is a very common explanation.
A baby's digestive tract is immature and does not digest as efficiently as an adult. If the mother is nursing, it could be possible that something she ate may be irritating the baby. Other explanations have nothing to do with diet. The baby may have been over stimulated during the day and therefore taking its frustrations out accordingly or the mother may be having some tension and the baby could be picking up on that. There are several ways to calm a colicky baby.