system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 Why can our bodies heal/repair broken bones, cuts, etc. but not repair the effects of aging?
gpt
I think most of the comments are dodging the deeper question of why cells are unable to divide and reproduce endlessly. While it's true that we evolved to include cell death, and evolved to die, there is an underlying biology that causes cell senescence.

The problem is damage to your cells' DNA. In order to repair any damage, your cells have to be able to produce the necessary proteins. Imagine keeping a 3D printer working by 3D printing the parts you need to repair it. As a bearing or nozzle begins to wear down, you can just print a new one and install it. In fact, if your 3D printer falls apart entirely, you can use your other 3D printer to print an entirely new printer.

However, in order to do that you have to have the files that the printer uses in order to print the parts you need. If those files get corrupted in one of the printers, it won't be able to correctly make the part that it needs. If the file is only a little damaged, the printer may be able to sort of make a new part that *mostly* works, but it isn't perfect. As you replace parts, even between different printers, the new parts will be made from corrupted files so the parts will be less and less able to work properly.

Why can't your cells repair their DNA? Your cells *do* repair DNA, a *lot*. However, their methods for detecting damage aren't perfect. Some damage inevitably manages to sneak through. It may *appear* to not be damaged by the proteins built to find damage. Or, the damage may occur on the parts of the DNA needed to *make* those proteins, so that cell can't detect the damage at all. Or, the genes you need to build the repair proteins may themselves get damaged and broken. Eventually, inevitably, damage accrues and your cells' ability to detect or repair the damage wanes, until the damage *cannot* be repaired and your cells have to make do with faulty proteins.

The damage comes mainly from three broad sources. The first is mutations caused by carcinogens and radiation. These things directly damage your DNA. As stated, your cells have a lot of ways to detect this damage and fix it, or to cause the cell to self-destruct so that it can't pass that damage on to a new line of cells. That can always fail, though. The catastrophic version of that failure is malignant cancer, but even if the cells don't become cancerous they may not be able to do their job effectively.

Another source is errors that occur during DNA replication. DNA is a *big* complicated molecule and the proteins that copy your DNA strands don't always work the way they should. You guessed it: your cells have tools to avoid that happening and tools to fix it when it does happen, but again, sometimes those tools fail and an error occurs. That error might insert, delete, move around, or change the base pairs as they get copied. One or both of the new daughter cells will take that error with it, and if that error damages a vital gene, the cell can't build that protein and may not be able to do its job.

The last source is a bit more complicated and has to do with how cells copy DNA. There is an enzyme that your cells build that "unzips" your DNA, and then another enzyme (actually two, it's complicated) comes along and grabs onto the end of it and "zips" it back up, using new base pairs for one side so that one side of the new strand is old base pairs and the other side is new base pairs. The enzyme that "zips" the DNA back up needs a bit of DNA to grab onto. It can't start at the *very* end. It's kind of like an actual zipper, that you need the little tab at the end to pull the zipper on.

That little end tab does *not* get copied by the enzyme. That means some of your DNA gets left behind and doesn't get copied. Your DNA has bits of DNA stuck to the ends called *telomeres* that exist solely to be grabbed by that enzyme and get left behind without getting copied to the new DNA. Since the telomeres don't code for anything, it doesn't matter that your new cells lose it. Your cells start with a *bunch* of telomeres and lose them over time. Eventually, cells that reproduce very quickly (like skin cells) run out of telomeres and start losing important DNA as it gets copied. When your cells are first dividing as an embryo, they produce an enzyme that builds those telomeres so you don't run out too early. At a very young age, though, the gene to make that enzyme turns off, the enzyme goes away, and you stop making telomeres. Why? Because, as stated by others, death is built into our evolution. There's no point in building endless telomeres as your risk of dying to disease, predators, or cancer increases with age. You have also evolved to die so that you don't continue to take resources from your offspring, which would decrease their chances of surviving and spreading your/their genes further.

TL;DR: Your cells need DNA instructions to build the things needed to repair themselves and everything else in your body. Your DNA gets damaged as you age until your cells don't have the instructions they need to build the things they need to repair damage.