Some people might be able to live independent lives at 18. Some people might be capable of independence at an even younger age. Certain people might need to live at home for a few extra years while they finish college. People who aren't as privileged in life might not even feel ready or capable to be on their own until their mid or late twenties or beyond.
Everyone matures at a different rate and age. Because of financial issues and the bad economy, many college graduates are even needing to move in with their families again. The concept of a nuclear family is a modern one, and perhaps it's a model that isn't practical in all circumstances. Depending on maturity, financial situation, and other factors, it can make sense to be dependent for longer than 21 years. However, one should always be striving to be a support to one's family (for example, taking care of the house, if not financially able to contribute), and not a burden.