The biology of aging starts at the cellular level.
NAD (short for nicotinamide adenine dinucleotide) is as essential a component of the individual cell as water. If it is absent the cell will die.
Each cell of our body has a nucleus and an external cell membrane. Outside the nucleus and within the cell membrane is a fluid called cytoplasm, and within the cytoplasm swims, amongst other organelles, these mitochondria (plural for mitochondrion). The mitochondria are responsible for the various activities of the cell.