Why do we shrink as we age?

Have you noticed someone getting shorter as the years slip by? Some people may start hunching over and even get a few inches shorter. So what makes us shrink as we age? 

It turns out that it's a combination of our bones "eating" themselves, our cartilage thinning and our muscles being whittled away. But the rates at which these processes happen vary depending on genes, physical nutrition and activity levels across a person's lifespan.

"We all age differently biologically," Marian Hannan, an epidemiologist at Harvard Medical School who researches aging, told Live Science. 

Nonetheless, people invariably get shorter as they age. A National Institute of Aging study that followed 2,084 men and women for 35 years found that they started losing height around age 30 and that the shrinking accelerated over time. 

The study, which included people ages 17 to 94, found that the men, on average, lost 1.2 inches (3 centimeters), and the women lost 2 inches (5 cm), between age 30 and 70. By age 80, men had lost 2 inches (5 cm), and women had lost 3 inches (8 cm). Largely, that's because our bones begin breaking down as we age. Bones start forming around the eighth week of pregnancy. They continue to grow until people reach their mid-20s. Bones also become denser when they have to support higher muscle mass. As muscle grows, it produces collagen fibers that stretch and increase local blood flow, which in turn stimulates bone growth. 

Bone growth plateaus by about ages 25 to 30. And around age 40 to 50, we begin to gradually lose bone mass, as our bones start breaking down old bone faster than the body can make new bone. 

Related: Scientists discover 4 distinct patterns of aging