Hi, so this isn’t my area of expertise but I don’t want your question to go unanswered! As I understand it, as we get older our bodies just become less good at repairing themselves. When we are young and we injure ourselves we heal very quickly. All the cells in our bodies are being replaced all the time, as we get older our bodies do this more slowly, they get worse at doing it. This means that damaged cell material builds up. In addition extra links between cells form and this does things like make our skin less stretchy (that’s why it goes wrinkly).
These things as well as some other processes that occur make it much harder for our bodies to replace dead cells with new ones and to repair damage. When we are older our bodies are also less good at repair, our cuts heal more slowly, that’s one of the reasons why it’s much worse for an older person to do something like break their leg, as it takes longer to mend.
So ageing is what happens as our bodies get less good at repairing themselves.