The life cycle of hair has three stages: anagen, catagen and telogen.
The anagen, or active, phase refers to the stage during which the hair grows continually. This phase normally lasts up to five year.
The catagen, or transitional, phase refers to the period of time in which the stoppage of growth of an old hair occurs and the growth of a new hair in the papilla begins. This lasts between two and three weeks.
The telogen, or resting, phase is when the new hair grows in the follicle while the old hair has become dormant. After ten to twelve weeks of this phase the new hair will force the old hair from the follicle.
The life cycle of hair causes up to 100 hairs to be shed each day. The average growth rate is about one inch a month. Hair grows fastest in the summer and slowest in the winter. Age plays a factor too. From the ages of 15 to 30, hair growth is most productive. When you reach the years of 40 to 50 the growth of hair slows down dramatically. Hair loss can begin at any age really, but most Caucasian men experience some loss in their 30s.