That is a very good question. In fact, it depends on the animal that you're talking about. Hair (or fur) is a distinct characteristic of mammals. We are mammals and so are almost all animals that have hair. Hair develops from a small structure called a follicle that grows in the skin and extends outwards. Many baby animals will have a full set of fur by the time they come out of the mother's womb. Often more hair will grow on the baby as they get older but the timing of when this occurs depends on the type of mammal. Humans are known to have much less body hair and often babies will have some hair on their head when they are born. Human hair grows on average 1/2 inch per month, so in a year, hair often grows about 6 inches. Thanks for your question!