About 1 in 5 babies cry long enough to be considered colicky. The timing varies, but colic usually affects babies beginning at about 3 weeks of age. It peaks somewhere between 4 - 6 weeks of age.
The crying associated with colic usually occurs at the end of the day. Babies with colic tend to be unusually sensitive to stimulation. Some babies have more discomfort from intestinal gas. Some cry from hunger, others from overfeeding. Some cannot tolerate certain foods or proteins in breast milk or formula. Fear, frustration, or even excitement can lead to colic symptoms. When other people around the baby are worried, anxious, or depressed, babies may cry more.
Colic eventually goes away. Symptoms begin to improve after about 6 weeks, and are generally gone by 12 weeks. If your baby still has colic at 12 weeks old, you should see a doctor to rule out another condition, such as reflux.