system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If our body replace our skin every 7 years, why does our skin start to look aged?
gpt
Your skin consists of 3 layers. Outside is the epidermis, these cells are indeed replaced, not even every 7 years, but every single month, so no problems there.

The layer under the epidermis is called the dermis. The dermis has cells, these are also replaced, not quite as fast as the epidermis, but still fast enough, still no problems. However, surrounding these cells are loads of filaments, the most important for the ageing effect are the elastic fibers. These cannot really be replaced, they were all formed when we were an embryo or in the first years of our lives and that's more or less it. So it is the loss and/or damage of these elastic fibers that gives us wrinkles and not the cells themselves.

(the 3rd layer is the subcutis, which is more or less the fat under the skin, but is also not really involved in ageing)

Contrary to some other comments, it's really not because of DNA damage or cell division.