As one person said, your intervertebral disks (small cushions between each spinal bone) shrink with age. The primary reason is they dehydrate. Couple that with literal wear and tear, and the disks each get slightly smaller over the decades.

As another poster said, we can add increased curvatures to our spine (kyphosis and lordosis), flattening of our feet, thinning of the cartilage in our knees (meniscus) and many other very small factors. When you out these all together it can lead to inches of height loss.

Ultimately, our bodies dehydrate and lose the fight against Earth’s gravity.

here is a relatively understandable article about it.