There are a few things that cause baldness, but the main reason is hereditary. If your father became bald at an early age, the likelihood that you will go bald is pretty high. Women are a little different. It is hereditary too. If your mother loses her hair, then there is a greater likelihood that her daughters will lose their hair too in the same fashion as the mother.
Another way for people to lose their hair is by frequently pulling your hair back into a bun or ponytail. When it is very tight and done a lot, hair can eventually fall out. This is usually a problem for women, but men often pull their hair back into pony tails and this may make them lose their hair at an early age.