system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If most cells in your body are recycled and replaced approximately every 7-10 years, why is there permanent damage from things like smoking?

I know that some cells are never replaced, but from what I am aware of, the time frame I stated is generally accurate.
gpt
1) The replacements are not airdropped in by the Cell Fairy, any cell you have is produced by another cell dividing. And division working as it does, DNA damage is inherited by the child cell if the progenitor is damaged itself.

2) There's more than just cells in your body. For instance, the reason skin sags with age is due to damage to the extracellular proteins of the dermis that normally support the actual cells, or things like arteries clogging. A lot of these cannot be helped by replacing cells, but may be damaged by external factors.