It does take a while for skin to heal (and it takes longer for older people). You have to wait for the biological processes that stop and bleeding and generate new skin cells. And in larger injuries, it doesn’t heal to give perfect skin: you can get inflexible scar tissue. Lots of scientists are trying to find ways of encouraging skin to heal quicker, and better. But that hasn’t been solved yet.
The process of healing does take different amounts of time in different people. It does tend to be much quicker in kids and slows down as you get older. This is all due to genetics and the genes that control the systems in the blood. Our cells (and therefore our genes) get copied all the time and as we get older this copying process gets mistakes in it. Its like taking a photocopy of a photocopy – over time the image gets a little fuzzy. The mistakes that we gather affect all sorts of things in our bodies, such as how fast we heal cuts.
Well, when we are cut the body has to do a lot of different things. Firstly, it has to detect that there was a cut, and the blood has to coagulate to stop the bleeding. Then it has to tell the nearby cells to grow and replace the cells that were cut. To do this, the cells need nutrients and oxygen, but this takes time because it they have to come from the blood, and the blood has just coagulated, and some of the blood vessels were cut along with the skin! So, the cells have to send signals that cause the existing blood vessels to get bigger, to let more blood in. Then, they have to send other signals to tell the immune system to pay attention, so that it can fight off any microbes that get in through the cut. Plus, the new cells have to be connected to blood vessels, so new blood vessels need to grow. On top of all this, the body has to disassemble any cells that died from the cut, and recycle all their components. All this takes time, so its pretty amazing that it can all happen in a few days!