Why does everyone decrease their height after certain age?

Osteoporosis. : A loss of height related to old age is related to osteoporosis and resultant vertebral compression fractures.

Loss of connective t: Shrinkage of the intervertebral discs. Some of them may also herniate. Osteoporosis may also exist and sometimes there are also compression fractures. Most people do not have compressed bones, even with osteoporosis.

Degenerative disc: disease causes desiccation of the intervertebral discs, which causes them to flatten, which decreases the distance between the verterbral bodies(disc space), which directly decreases height. Other factors that can contribute to height loss are spinal alignment changes(kyphosis), and possible compression fractures due to osteoporosis