Hair thinning and loss is a normal part of aging for many people. As you age, your body produces less keratin (the protein hair is made of), resulting in slower hair growth and regrowth, sparseness, and hair loss.
Genetics is the largest factor that predisposes you to early hair loss and thinning. If you have family members who experienced early hair loss, your chances of early hair loss can significantly increase.
Other lifestyle factors like medical conditions, hormones, medication, excess stress, and tight hairstyles can cause hair loss.