You can find information about the hair cycle
here
Some relevant paragraphs: “Hair cycle includes anagen (the growing phase), catagen (the intermediate phase) and telogen (the shedding phase)… Anagen lasts between three and seven years in most people.”
“As people grow older the period of anagen shortens. For example, the hair of someone with a five-year anagen can grow to a length of 60 cm before it enters the shedding phase. If their anagen period drops to three years as they age, their hair will then grow only to shoulder length before it falls out or is brushed out. So when a scanty-haired elderly lady boasts that when she was a girl she could sit on her hair, she may very well be telling you the truth!”
“Over the years, the number of follicles capable of growing hair declines naturally. The decline is especially noticeable on the top of the head. Some follicles increasingly produce only fine, short non-pigmented hairs that look more like vellus hairs than terminal hairs. In older women, this leads to a general thinning of hair. In men it tends to lead to common baldness. If you look at a bald scalp you will see these fine, poorly pigmented hairs.”
