It is thought that the bit of the cell that helps the DNA divide shorten and become less able for cells to divide and replace damaged cells. So, this leads with wear and tear to damage which mean that parts of our bodies wear out.
Cells replicate throughout our lives, with this rate of replication effected by lots of environmental factors. All cells will have certain lifespan beyond which they can no longer grow. So once they reach this point cells will begin to die, so this would lead to gradual tissue and organ failure over time…..