Because over time the replication of cells becomes imperfect, I'm not going to try and explain it because I don't feel like I truly understand it but that's the

Because over time the replication of cells becomes imperfect, I'm not going to try and explain it because I don't feel like I truly understand it but that's the essential bit, after around the age of 35-40 your cells stop replicating perfectly and errors start building up which eventually lead to the actual cause of death, things like organ failure or cancer. Thanks for catching the typo, I don't look these posts over well enough sometimes.