Great question. There are a few reasons, but they all basically come down to our bodies working more slowly and cells not being replaced as fast, or not being replaced at all.

Our bodies are made of cells and the things cells produce (like hair or bones). When we’re young, cells are dividing quickly. That’s how we grow. But cells only divide a certain number of times, and things start to slow down.

We get shorter because our bone cells are fewer and slower, so they aren’t making new bone as fast. The bones get weaker and just the pressure of our bodies squashes them a bit, so we lose height. The weaker bones can also keep us from standing up straight, so we look shorter.

We get wrinkly when our cells make less of the types of proteins that make our skin elastic (stretchy) and make it thicker so that it doesn’t wrinkle. We also replace skin cells more slowly.

What other changes have you noticed when people, or other animals, get older?

We need to use the element calcium for our nervous systems, and store it in our bones. For reasons that I don't understand and am not sure are understood, old people can't take up calcium from food as well as young people, so they take the calcium in their bones out and don't put it back. Since calcium is also what makes bones stand up, taking out the calcium makes them shrink, which is why old people shrink.

It’s true that people tend to get shorter as they get old, with the effect being worse in women. One study found that women lost about 3 inches in height from age 30 to 80! Most of this height loss has to do with the spine. The spine is made of a bunch of small pieces called vertebrae. As people get older, the vertebrae get squeezed to be slightly shorter which makes the person shorter. Also, the loss of bone density and muscle can make a person stoop over which will make them look shorter.

Height loss is a normal part of aging, but sometimes when it’s too fast, it may mean there is something wrong with the person.

We can shrink for a few reasons.
1) Our cartilage between joints wears down, and osteoporosis, which is a weakening of the bones that can lead to increased risk of bone fractures, shortens the spinal column.
2) We lose muscle and begin to stoop. Therefore, it is important for us to exercise an appropriate amount every day to maintain our muscles and bones, and keep our joints in good health =).

This is a good question. As we get older, the discs (fluid-filled sacs) in our vertebrae (bones in our spine) dehydrate (lose water) and compress (get smaller). This means that overall, we shrink. The next question you may ask is why these sacs dehydrate over time. Our discs have molecules called proteoglycans, which usually act to attract water. As we age, the amount of proteoglycans decreases in our vertebral discs. While we can't totally prevent the process of aging, we can do certain things to slow it down. Sleeping and eating well, staying hydrated (drink more water, less coffee!), and stretching/exercising to keep your back healthy can help keep your discs more hydrated over the years. Hope this helps!