The majority of the process is controlled and purposefully done by the DNA. Your body grows in stages and goes through major changes from embryo, to fetus to infant, toddler teenager etc. Your body is developing mostly as intended. As we age though, our cells' DNA slowly breaks down over time and as cells continue to use DNA to perform lifer functions like making proteins or cell reproduction, the DNA is compromised over time and the cells cannot perform their functions as well and slowly perform worse until they start causing stress on cells around them and so on until a major bodily function fails.

here's a not very satisfactory explanation that i found when playing with genetic algorithms at uni:

if you don't age, you end up competing for resources with your offspring. the best way to win that competition is to generate offspring that are worse than you. this leads to genetic stagnation and makes your population vulnerable to changes in the environment.

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.

The cells in our body are constantly replicating themselves. During the replication process, DNA is copied and pasted for the new cell. The copying and pasting process doesn’t always go right and sometimes you run into mutations. Aging is a part of this DNA mutation phenomenon. As more things go wrong, your body slows down and becomes weaker. This along with all the physical stress your body goes through on its life.