Most of the different types of cells in our body die and are replaced every few weeks or months. However, neurons, the primary cell of the nervous system, do not multiply (for the most part) after we are born. That means that the majority of the neurons in your brain today are as old as you are. This longevity of the neurons partially accounts for why we feel pretty much the same on the inside at the age of 10 as we do at age 30 or 77.