system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is holding back science from enabling us to have eternal life?
gpt
As cells in our body die, they're replaced by the body reading our DNA to recreate the cells. However, every time a strand of DNA is 'read', a little piece of it gets used up (like a tail with the end getting clipped). Once it's clipped off, there's no getting it back, and once the tail is clipped completely, the DNA is unreadable. 

This process (which I've GROSSLY oversimplified and am probably not even explaining properly) is what makes us age, grow and look old. It's why organs can start to fail and we just end up dying when we can't produce our cells properly.

So far, science hasn't figured out a way to stop this process.