Our cells continue to make copies all the time. Whenever a copy has a problem - a mutation - then the cell usually kills itself. Sometimes it doesn't and the mutation lives on. Some of these aren't a problem but some are. For instance, cells can grow too quickly while being mutated and that's called cancer.

Getting old is basically our body not being good at copying cells anymore where a lot more errors are introduced to the point where our body eventually shuts down. This isn't a bad ELI5 of aging.

We're not good at solving the problem though. There's some research in stem cells but that might be more like applying duct tape to torn clothes rather than making clothes repair themselves or last longer.

Think of it as making a copy of a copy. We can create printers that last longer (healthy diet, etc) but in the long run, it's still a machine and machines break.

There is another special type of cell that's required for life: neurons. Unlike other cells, neurons don't divide. So right before you're born, you have the most neurons you'll ever have in your life and you'll continue to lose them as you age. Again, stem cells could help here via transplants which is an artificial solution at best.