It depends on how you think about it. If you think about it from the perspective of your whole body, then yes. Cells live, and divide to become new cells, and die. So every day this process is happening in your body and you have new cells that you didn't have the day before, and other individual cells that have disappeared. But that is only a small fraction of your total cells. Individual cells in your body can exist for just a day, or for your whole life. There are many different kinds of cells in the human body and each type has a different average life span. Nerve cells may last a lifetime, red blood cells can live about four months, and skin cells or the cells that line the inside of your mouth may live only a few days. Cells can also be affected by things like chemical and temperature, which could require them to be replaced faster or more often. The oldest example of a living cell is currently bacteria taken from a rock that was 250 million years old. Scientists found the bacteria cell living in a state of "suspended animation" in the rock - when they brought it to the lab, it came back to life and started growing again!