That's a great question! It turns out that the idea of cells never being replaced is a bit of a myth. While some cells in your body have very long lifespans and are rarely replaced, all cells in your body are technically capable of being replaced throughout your life.

However, there are some cell types that are considered "post-mitotic" which means they don't divide and are not easily replaced if they die. These include:

* Neurons (brain cells): While there is some evidence of limited neurogenesis in certain brain regions, for the most part, neurons are not replaced when they die. This is why brain injuries and neurodegenerative diseases can be so devastating.

* Heart muscle cells: While some research suggests limited regeneration, heart muscle cells are mostly considered post-mitotic. This makes heart attacks particularly dangerous as the damaged muscle cells are not easily replaced.

* Lens cells of the eye: These cells are formed during development and are not replaced throughout life. This is why cataracts, a clouding of the lens, can occur.

It's important to note that research is constantly evolving and new findings may reveal more about the regenerative capabilities of these cells.

So, while some cells in your body have extremely long lifespans and are rarely replaced, no cell is truly "never replaced".